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This illustration is taken from “The Life of Rekhmara” by Percy HE. 
Newberry (London, 1900), plate XVII, and is the sculpture referred to by 
Wilkinson (see p. 14). It occurs on the walls of the tomb of Rekhmara 
at Thebes. 

Newberry states (p. 36): “In the third row are shown the carpenters, 
wood carvers, cabinet makers, and painters. The carpenters saw rough 
wood up into planks and prepare it for the cabinet makers, who are repre- 
sented making chairs, boxes, a slender wooden column with lotus-bud capi- 
tal, and an elaborate shrine inlaid with ivory and precious woods. . . . To 
the right of this shrine, beyond the men making a chair or couch, are two 
ae working with glue which is being heated in a pot over a charcoal 

rere 

The portion of the engraving showing the glue pot heating, is given, 
partially in the original brilliant colors, by Rosellini, “Monumenti dell’ 
Egitto e della Nubia,” Vol. 2, Monumenti civili, plate XLVI. 


GLUE AND GELATIN 


BY 
JHROME ALEXANDER 


AUTHOR OF ‘‘COLLOID CHEMISTRY, AN INTRODUCTION ’’ 





- American Chemical Society 
Monograph Series 


BOOK DEPARTMENT 


The CHEMICAL CATALOG COMPANY, Ine. 
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1923 





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The CHEMICAL CATALOG COM 





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THE GETTY RESEARCH 
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GENERAL INTRODUCTION 


American Chemical Society Series of 
Scientific and Technologic Monographs 


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4 GENERAL INTRODUCTION 


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GENERAL INTRODUCTION , 5 


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PREFACE 


The manufacture of glue and gelatin is important not only 
because of the magnitude of the industry itself, but also because 
these products are essential to the production of many others, 
thus making it a key-industry. Furthermore glue and gelatin 
are typical jelly-forming colloids and they have been used in 
numberless experiments on and investigations into the nature 
and behavior of colloids. As a consequence there is an embar- 
rassing wealth of publications from which to draw material for 
a monograph. For the same reason, a discussion of the behavior 
of glue and gelatin involves the consideration of many moot 
points in colloid chemistry, and gives the subject an interest 
rather broader than the title would indicate. 

The theoretical aspect has been treated more at length than 
has been usual with books on glue, for a more complete under- 
standing of the nature of a product must in the end be useful to 
its makers and users. Where opinions vary, the different views 
are given, often in the very words of their principal advocates. 
Nor have I withheld my own views in such cases. 

In the technical sections elaborate descriptions of well-known 
apparatus have been avoided, because new forms and modifica- 
tions are continually appearing, and any one can have the latest 
particulars from their manufacturers. Such descriptions use 
space to no good purpose, and may give a reader at some later 
date the erroneous idea that the machines described are then 
the best of their kind. However the principles involved in 
manufacture, testing, and use, have been particularly stressed. 

On the other hand many excellent papers have been omitted 
or referred to but briefly, some because the points involved were 
treated adequately by others, some because they are beyond the 
scope of this book, and some perhaps because of inadvertence 
or inaccessibility. No attempt has been made to pass on ques- 
tions of priority, and the fact that a certain author is quoted 
as expressing certain views, does not necessarily imply that he 
was the first to express such ideas. For the benefit of those 

7 


8 PREFACE 


who wish to look further into such papers, reference may be 
made to the bibliographies and indexes mentioned below. 

I am indebted to many authors for books and reprints, and 
to a still larger number for scattered items of information. In 
all cases I have striven to make due acknowledgment in the 
text for material used, and to refer wherever possible to the 
original sources of information. 

JEROME ALEXANDER. 


Bibliography. 


An extensive bibliography on Glue, comprising about 150 titles, 
was published by Rudolf Ditmar in the Kolloid Zeitschrift, 
1906, Vol. 1, p. 80. Another bibliography of about the same 
extent was published by Robert H. Bogue in Chemical and 
Metallurgical Engineering, 1920, Vol. 23, No. 5. The First Re- 
port of the British Adhesives Research Committee contains a 
descriptive bibliography of gelatin (75 pp.) by T. Slater Price, 
which gives a resumé of much important work. 

The indexes of the principal chemical, physical, biological, 
and technical journals may be consulted, and it must be remem- 
bered that many experiments with gelatin are apt to be found in 
papers or texts indexed under such headings as “Proteins,” ‘“Col- 
loids,” ‘Jellies,’ “Diffusion,” etc. The collective indexes of 
Chemical Abstracts, of the Journal of the Society of Chemical 
Industry, and the indexes of the Kolloid Zeitschrift will be found 
particularly useful. 


CONTENTS 


PAGE 
Cuapter 1. Introduction . . faa heli 19 
Definitions. —Philology Trees ewetie and gela- 
tin industry in the United States.—Statistics of glue 
and gelatin industry in various countries. 


CHAPTER 2. eeeiicn of Gelatin among the Proteins, and 
the Nature of the Force ‘Binding fogstner its Con- 


stituents . . eke 229 
Classification of Bens _ ary etree — Conju- 
gated proteins.—Derived proteins—Molecular struc- 
ture. 


CHaprter 3. Chemistry, Physical Chemistry and Colloidal 
Chemistry of Gelatin and Glue . . . . . 30-50 


Chemical structure.—Loeb’s Theory.—Ash-free gelatin. 
—Fischer’s views.—Molecular weight.—Crystallization 
of gelatin. 


CuHaptTer 4. Chemistry, Physical Chemistry and Colloidal 
Chemistry of Gelatin and Glue... . ... 51-67 


Is gelatin a distinct chemical entity ?—The significance 
of hydrogen ion concentration.—Titration curve of gel- 
atin. 


Cuapter 5. The Structure of Gelatin Solutions and Gela- , 
eee eliieds 6 60-5 Tt hy sie Pate Pag a ate 68-86 


The Ultramicroscopic Ey ates 


CHapter 6. The Influence of Various Factors on the 
Swelling of Gelatin . .. pS eo ees 87 


Lyotrope or Hofmeister series. MET oanane Theory. — 
Thermal expansion of gelatin. 


ee 7. The Viscosity of Glue and Gelatin Solutions 
98-112 ° 
Influence of added substances on viscosity. 


CHapter 8. Collagen or Ossein . . .. . . . 1138-120 
-Chondrigen, chondrin and mucin. — 


9 


10 CONTENTS 


PAGE 
Cuaptrer 9. The Effect of Tanning Substances on Glue 
ANG UAREIA GING ta pas ec. Se ete 


Tannin.—Chrome.—Organiec substances——Bichromates. 
—Silicie acid—Alum.—Iron.—Other salts—The halo- 
gens.—Formaldehyde. 


CuaptTer 10. Chemical Examination of Glue and Gela- 
151s CCU MR EMEC rar TNT ee 
Hydrogen ion concentration.—Total acidity.—Ash.— 
Determinations involving nitrogen.—Diffusible nitro- 
gen test.—Reactions of gelatin.——Detection of glue and 
gelatin in various chemical products——Gold number. 
—A,. O. A. C. methods. 


CuaptTer 11. Technology of Glue and Gelatin Tek. 151-172 


Glue stock.—Treatment of glue stock—Apparatus and 
methods.—Bone stock.—Hide and sinew stock.—Wash- 
ers—Tanned stock.—Boiling apparatus and methods. 
—Open kettle or tank.—Pressure tank.—Clarification, 
bleaching and evaporation of dilute liquors.—Antisep- 
tics—Chilling—Cutting, spreading and drying.—Blow 
down processes.—Percentage yields of glue stock. 


Cuapter 12. The Testing and Grading of Glue and Gela- 
tin 2 ee 4 rr 
Jelly strength—Viscosity or Running Test.—Water 
absorption test——Hygrometric test.—Melting point.— 
Setting point.—Strength test——Laboratory test series. 
—Standard glues——Smith’s Polariscopic method.— 
Moisture.—Ash-Recording tests. 


CuapTer 13. Uses of Glue and Gelatin. . . . . 200-215 
Adhesive.—Sizing and stiffening —Compositions.—Col- 
loidal protector or colloidizer—Miscellaneous.—W ood 
joints.— Veneers.—Paper boxes.—Leather belting.—Siz- 
ing.—Printers’ rollers, etc.—Photography.—Inhibiting 


crystallization. — Bacteriology. — Formogelatin. — 
Gelatin as a food.—Food vs. technical gelatin, 
Cuapter 14. Fish Glue and Fish Isinglass . . . 216-223 


Origin —Properties of fish glue—vVarieties of isinglass. 
—Properties of isinglass.—Uses of isinglass. 


AppENnpDIx—Test Methods of the National Association of 
Glue and Gelatin Manufacturers >. > Spies 


AuTHOR INDEX . . ) .°.) woe 
Supyect INDEX .. ..° : 5%) 204 09 See 


GLUE AND GELATIN 


Chapter 1. 


Tntroduction. 


Definitions. 


Glue is an organic colloidal substance of varying appearance, 
chemical constitution and physical properties, obtained upon 
drying the solutions resulting from boiling with water properly 
prepared animal matter such as skin and bone. The jellies 
which form on chilling soups, stews, boiled chicken and the like, 
represent very impure glue solutions. 

Glue appears in commerce in a wide variety of forms and 
colors, some of which are commonly, but erroneously, believed 
to be criteria of quality. The colors range through all shades 
of white, brown and yellow, and it may be transparent, translu- 
cent, or opaque. Gelatin colored red with aniline or vegetable 
coloring matter is used as a top dressing for cold meats, and 
specially colored glue compositions are used for making paper 
pads or blocks, ete. 

In European countries glue is usually marketed in the form 
of oblong cakes about 3x6 inches (Cologne shape) or in sheets 
about 10 inches square (French glue). “Scotch” glue comes 
in very thick cakes, about 6x10 inches, with a string through 
one end. In America the bulk of the glue is used in ground 
form, though considerable is sold in thin, broken flakes, and 
' some is made in the form of “noodles” or “ribbons,”’ which spe- 
cial forms have no advantage other than a higher cost. Pow- 
dered glue is also used principally in mixing with whiting to 
make calsomine. For Eastern countries there is made a form 
known as “bazaar glue,” which consists of a poor quality of glue 
in square sticks about 8 inches long and 1 inch in section. 

Gelatin is made from bones and skin or hide fragments, 

11 


12 GLUE AND GELATIN 


selected, cleaned and treated with especial care so that the result- 
ing product is cleaner, purer and generally clearer and lighter 
in color than glue. Glue is in fact impure gelatin, and any glue 
possessing suitable strength and appearance may be termed 
gelatin, although of course all gelatins are not suitable for food 
purposes. 

It is surprising that such authoritative reference books as the 
Standard Dictionary and Murray’s Oxford Dictionary perpetu- 
ate the popular error that hoofs and horns yield glue. Hoofs 
and horns consist of keratin and are always removed by the 
glue maker, although of course the feet of animals and the 
interior bony support of the horn (horn pith) yield glue or 
gelatin (e.g. calves’ foot jelly). 


Philology. 


The word glue has been traced back to the unused Latin verb 
qluere, meaning to draw together, and the form glus, glue, was 
used by Ausonius. It is allied to the Latin gluten, glue, and 
glutus, tenacious (cf. the English gluten, glutinous), to the 
Greek gloios, mud, gum, and to the old French glu, birdlime. 
Birdlime is a sticky substance which exudes from the holly tree, 
and is used for snaring birds. In this word lime has the signifi- 
cance of the German leim, glue, being in no way related to lime, 
calcium oxide. The word gelatin comes from the Latin verb 
gelare, to congeal (cf. the Latin gelu, frost, and the English chill, 
gel, gelid, jelly). Thus Virgil in describing the awe inspired in 
the Trojans by the Cumzan Sybil, says: 


... Gelidus Teucris per dura concurrit 
ossa tremor . 


The word gelatin came into the English through the French 
gélatine, from the Italian gelatina. Gelatin is often termed 
glutin by chemists, a practise that should be abandoned, owing 
to the similarity of this word to gluten, the composite protein of 
rye, wheat, etc. Gelatin is often erroneously termed “isinglass,” 
the confusion being due to the fact that gelatin under the name 
“patent isinglass’” came into use as a substitute for the true 
isinglass (see p. 219) and resembles it in appearance and working 
properties. Popular error goes so far as to apply the term 


INTRODUCTION 13 


isinglass to the mineral mica, which also appears in thin, trans- 
parent, flexible sheets. 

Since the essential meaning of glue is that which draws or 
sticks together, while gelatin means essentially that which gela- 
tinizes, it is but natural that, in popular parlance, the use of 
these words have, by similitude, been extended to many sub- 
stances which are not glue or gelatin at all. Thus solutions of 
gums, dextrins, converted starches, etc., are often called glues, 
the modifying adjective “vegetable” being generally used. Sili- 
cate of soda is sometimes termed ‘‘mineral glue,” solutions of 
rubber, pitch and the like are called “marine glue,” and those of 
casein are called “casein glue.’’ Several varieties of sea weed, 
including Gelideum corneum?‘ or agar agar, form, when cleaned 
and dried, a cord-like product having enormous gelatinizing 
power, which appears in commerce under various names, such as 
vegetable or Japanese gelatin, vegetable or Japanese isinglass, 
Chinese moss, gelose, etc. Our well-known fruit jellies, when 
pure, contain no gelatin, their gelatinization being due to a 
jelly-forming carbohydrate known as pectin. 

The correct uses of the terms above discussed are not always 
academic. They are often of importance in determining the 
operation of tariffs and other legislation. Thus the regulations 
of the Official Southern and Western Freight Classification (Rule 
14, section 2): “Fiberboard and Pulpboard used in making 
Fiberboard or Pulpboard boxes, without frames, must be three- 
ply or more, all plies firmly glued together .. .” The ruling of 
the chemist in charge was that the word “glue” meant glued 
with animal glue. This narrow construction of the rule still 
stands, although boxes meeting the tensile and other require- 
ments are passed, even if they are not glued with animal glue. 


Historical. 


Many of our most important discoveries have come as the 
result of some keen mind noting an incidental or accidental 
result. Though we may doubt the correctness of Charles Lamb’s 
story as to the origin of roast pig, discovery of British gum and 
dextrin is said to have followed the observation that some 
starch, which had been roasted or torrified during a fire in a 


1Gelidium gracilaria yields a similar product. 


14 GLUE AND GELATIN 


Manchester warehouse, yielded a sticky, gummy solution when 
wet with water. 

In all probability the discovery of glue grew out of the fact 
that stews, especially those containing bones or skins, yield a 
sticky solution and gelatinize when cold. It is to Egypt that 
we must look for the oldest record of the use of glue, the dis- 
covery of which evidently antedates the Exodus, as may be 
seen from the following quotation taken from Wilkinson.? 

“Among the many occupations of the carpenter, that of veneer- 
ing is noticed in the sculptures of Thebes, as early as the time 
of the third Thothmes, whom I suppose to be the Pharaoh of 
the Exodus; and the application of a piece of rare wood of a 
red colour, to a yellow plank of sycamore or other ordinary kind, 
is clearly pointed out. And in order to show that the yellow 
wood is of inferior quality, the workman is represented to have 
fixed his adze carelessly in a block of the same colour, while 
engaged in applying them together. Near him are some of his 
tools, with a box or small chest, made of inlaid and veneered 
wood, of various hues; and in the same part of the shop are two 
other men, one of whom is employed in grinding something with 
a stone on a slab, and the other in spreading glue with a brush. 
_ “Tt might, perhaps, be conjectured that varnish was intended 
to be here represented; but the appearance of the pot on the fire, 
the piece of glue with its concave fracture, and the workman 
before mentioned applying the two pieces of wood together, satis- 
factorily decide the question, and attest the invention of glue* 
3,300 years ago. This is not, howéver, the only proof of its use 
at an early period, and several wooden boxes have been found 
in which glue was employed to fasten the joints.” 

The manufacture of violins and similar musical instruments 
during the Middle Ages and Renaissance, especially in Italy, 
indicates that glue was known and used at that period, and there 
are indications that early painters used glue size in preparing 
their canvases. 


*Sir John Gardner Wilkinson, ‘““Manners and Customs of the Ancient Hgyp-. 
tians,” John Murray, London, 1879, Vol. 2, pp. 198-199. 

3 Rosellini seems to think that the application of color is here represented ; 
but the presence of the pot, containing the brush, upon the fire, will scarcely 
admit of this, though the figure grinding on the slab might appear to strengthen 
his conjecture. He has placed this subject with the painters of Beni-Hassan, 
but it is at Thebes. Pliny ascribes the invention of glue to Dedalus, as well 
as of the saw, the axe, the plumb-line, and the auger. (Plin., vii, 56.) 


INTRODUCTION 15 


Murray (New Oxford Dictionary) gives a number of refer- 
ences to the early use of the word glue by English writers. Thus 
in the “Squire’s Tale” (line 174), Chaucer (about 1386), in 
describing the wonderful brass horse on which a royal messenger 
appeared, says: 


“The horse of brass that may not be remewed, 
It stant as it were to the ground yglewed.” 


Further in Lanfranc’s “Chirurgeon” (about 1400) it is stated 
(p. 185): “As it were two bordis weren ioyned togidere with cole 
or with glu.” 

Glue and gelatin, like most other manufactures of early days, 
were produced by individual artizans for their own use, and 
even to-day some paper and textile mills boil their own glue size 
from rawhide cuttings. From these somewhat primitive methods, 
the real glue and gelatin industry emerged about the beginning 
of the nineteenth century. In France the industry started in 
the vicinity of Lyons, and for many years these factories were 
the most important of their kind in Europe. During the Napo- 
leonic era extravagant claims were made as to the food value 
of gelatin, and probably this was one reason why the industry 
was fostered. ) 

Germany apparently appreciated the importance of the manu- 
facture of glue and gelatin as a key industry, for a German 
company organized in 1895 with three plants, expanded until in 
1912 it controlled the output of seventeen plants, and had also 
factories in Austria, Russia, Belgium, Switzerland and France. 


The Glue Industry in the United States. 


According to Rufus W. Powell?‘ it is very difficult to obtain 
exact information about the glue manufacturing industry of the 
United States prior to 1860; but those long in the business re- 
ported that outside of regular glue manufacturers, a great many 
tanners boiled up their own stock in open kettles. The prin- 
cipal factories seem to have been in the vicinity of Boston, New 
York, Philadelphia and later on Cincinnati. One of the pioneer 
factories was at Marblehead, Mass., and probably secured its 
stock from the tanneries at Salem and Lynn. Peter Cooper’s 


4“Glue Statistics,’ Brooklyn, 1893, 


16 GLUE AND GELATIN 


factory was on Newtown Creek, Long Island, now in the Bor- 
ough of Queens, New York City. 

Powell gives the following table based upon reports given the 
Glue Manufacturer’s Association, 1887-1888, which accounts for 
the commencement of the 92 factories then reporting. 


1830 1840 1850 1860 to 1870 to 1880 to 


Location of Before to to to 1870 1880 1887 
Factories 1830 1840 1850 1860 Hide Bone Hide Bone Hide Bone 
New England ... 26 2 1 1 3 4 6 3 2 3 1 
Middle States ... 35 2 1 1 1 8s — 7 3° -10 2 
Western States ..24 — — — 2 5 1 6 2 6 2 
Pacific Coast ... 7 —2 — — — 1 — 3 — 3 — 
GOL ance eee 4 2 2 6 18 7. 239 rb ery. 5 


The Census of 1880 showed that there were then 82 plants pro- 
ducing glue as a principal or by-product. They employed 1,801 
hands and a capital of $3,916,750. 

Powell estimates that the total production of glue in the 
United States for the fiscal year 1886-1887 was 38,032,000 lbs., 
of which 27,743,000 lbs. was from hide, fur and neat’s-foot stock, 
and 10,289,000 lbs. was from bone, bone liquor, and pigs’ feet. 

Some idea of the range of glue prices during this early period 
may be gleaned from the following table abridged from Powell 
(prices given in cents per pound). 


1844. 1888 

1848 1860 1863 1867 1876 = 1887 1892 
A WExGra) 2 os sana 40 39 37 60 38 25 23 
Val Os cay Gah # 34 30 32 53 30 22 19 
NGEDISe. acta ee 30 26 27 47 30 19 17 
 orryainy eg ac ayer 25 24 24 41 25 17 15 
Aa Ripa eee 21 22 21 36 21 16 144% 
Les ic ay biaatn are 19 20 19 32 is 15 1 
La aa ee ae aes 18 19 18 29 17 14 13 
{he an Sn IPR irc 17 18 17 27 15 12 11 
Lge ce sae ne ce 16 17 16 25 14 11 10 
Lig NOS i aneeets — = — — — — 9 
Orbaky hehe dare ase: 14 16 15 23 13 10 8 


The total value of glue and gelatin produced in the United 
States for 1914 was $19,725,703, of which 40,844,650 lbs. valued 
at $3,088,764 was produced by the packing houses. The 1919 
figures given below do not include 36,603,000 lbs. of glue valued 
at $4,489,774, produced by the meat-packing industry, but may 
include some gelatin. No separate figures are obtainable for 
gelatin, and that produced by the packers is included under a- 


INTRODUCTION 


heading “All other products.” 


17 


Various other industries inci- 


dentally produced glue to the value of $1,039,794. 


The United States Census gives the following 


tabulated in- 


formation regarding glue, not elsewhere specified: 


is 
Ra's 
= S 3 rh 
ec 8 
Peso cee 
ois She S > 
SS as §3 
> a) eS 
Year and State =SZES as 
United States 
TOLGrite Saw e vaca ee 62 4,264 16,979 
lL 3 Sa oe eee 57 3,129 13,304 
S01! Aa eee 65 3,265 15,596 
RR tay ses 58 2,864 14,280 
Deena, viese ss 61 1 618 6,806 
Ee ee 62 | 697 4,912 
(seer Bo eLs0r 
TSO oe ivatsiene be bees» 70 ~=6.860-~—«i1,051 
PRbU ee is iss 62 875 — 
SL hes Feed ne eda 7 39)  —— 
States, 1914 
ADRES cals cs 9 968 3,316 
IOUT. Tine « <.'e 3 ral 355 
Massachusetts .. 11 563 1,481 
New York..... 9 381 2,082 
Pennsylvania ... 8 6519 1,628 
All other States. 17 627 4,442 


3 

e 

ees on Sania 5 
eS Sa Sar oe 
Wien we te Ra SS 
Serene ie SG wSs BEF 
23 Ruston Gane es Sty SS iss 
So ee Sat ht eS eS SS 8 
SS) SS Pas sks ses 

8 eS ee Ss 
So ie eg el ales och aot at et SS 
27,237 4,777 19,280 32,134 12,854 
17,162 1,854 9,868 13,733 4,365 
14,289 1,571 7,525 13,718 6,193 
10,673 1529 6186 10035 3,849 
6.144 685 3767 5,387 1,622 
PB50 D610, 2,0Lk 4.270. 1759 
3917 600 2786 4324 1,538 
1,955 310 883 1,710 827 
1,053 306 Doli ekg k SO 649 
520 99 372 652 280 
5652 614 2385 3,751 1,346 
356 30 see 280 123 
2,956 294 1,789 2,589 800 
2,459 249 1,942 2,483 541 
2320 290 1418 2029 611 
S019" 2337. 1677 ~~ 2621 944 


The United States Department of Commerce has kindly sup- 
plied the following figures regarding the imports and exports of 
glue, gelatin, and glue stock, which indicate that large amounts 
of foreign glues and gelatins are consumed here. 


1921 2 

Imports Pounds Dollars Pounds Dollars 
Gelatin (unmanufactured) 2,396,645 1,231,035 2,527,198 997 896 
Glue and Glue Size...... 3,561,831 762,557 4,174,785 574,311 
Hide cuttings, raw, and 

other Glue Stock...... 36,104,659 2.272847 25322414 1,149,883 
Exports (domestic) 
SS! 5 eS ee ea 5,991,872 1,148,666 2,101,328 348,643 


These figures indicate the well-known fact that different vari- 
eties of glue are made in different factories, and move according 


to their fitness for certain uses. 


18 GLUE AND GELATIN 


Great Britain. 


There are 57 glue manufacturers and 21 gelatin manufacturers 
in the United Kingdom, but figures regarding production are not 
available. For 5 months ending May 31, 1921, the following 
figures were kindly furnished by Mr. L. E. Bernays, British 
Consul at New York. 


Cwts. Value,& 

Imports—Glue, Gelatin, and Size......... 33,666 226,912 - 
Exports—Gelatin «, 0s (62 a whe cee ee 1,676 33,673 
Glue: and. ‘Size. ice fan ee 14,808 61,678 


France. 


In France there are about 60 factories, the main centers being 
Paris, Lyons, Marseilles, Dijon, LaPallice and Nantes. Ap- 
proximately 3,000 workers are employed. Before the war France 
produced about 11,000 metric tons of pressured bone glue and 
gelatin, 4,000 tons from acid treated bone, and 3,000 tons from 
by-products and waste. About 100,000 tons of bones were em- 
ployed annually. 

The following figures were obtained through the United States 
Department of Commerce. 


4 


1918 


Imports Exports 
Metric Value Metric Value 
tons (francs) tons (francs) 
Fish? glue fievaaeGht, wate ae Tia 2,020,200 118.1 3,070,600 
Glue from bones and other 
anitis |Swasie > sien cee hee, 1,883 2,259,600 7,367 8,840,400 
Gelatm in powder sheets, etc. 237.6 653,400 461 1,267,750 


In 1918, 20 tons of fish glue were imported from the United 
States, and 9 tons were exported to that country. Exports of 
bone glue to the United States amounted to 414 metric tons, 
while exports of gelatin to the United States were approximately 
54 metric tons. 


1920 
Imports Exports 
Metric Value Metric Value 
| tons (francs) tons. (francs) 
Fish. @lie vat eo eee 90.1 858,000 422.9 26,960,000 
Glue from bone and _ other 
BM al Washe-tan, tome paler 1,266.8 5,882,000 4,394.6 21,755,000 


Gelatin in powder sheets, etc. 83.6 - 1,484,000 765.8 4,545,000 


INTRODUCTION 19 


Germany. 


As before remarked, the glue and gelatin industry of Germany 
is a most important one, and has recently interested American 
capital. Besides supplying the large home market, an extensive 
export business is done. Figures are not at present obtainable. 

Belgium, Switzerland, Holland, and other countries also pro- 
duce glue and gelatin, and notable quantities are produced in 
Japan, Argentina, Canada and Australia. 


Chapter 2. 


The Position of Gelatin among the Proteins, and the 
Nature of the Forces Binding Together Its 
Constituents. 


Glue and gelatin belong to that large and important group of 
nitrogen-containing colloidal organic substances known as the 
proteins, which are found in nature as essential components of 
plants and animals and as products of their metabolism. More 
specifically they belong to the sub-group of proteins known as 
albuminoids by American and Continental chemists, -and as 
scleroproteins by the Chemical and Physiological Societies of 
England, because the group includes the substances which are 
the chief organic constituents of the animal skeleton and of the 
skin and its appendages, i.e. elastin (from tendon), collagen 
(from bone and hide), and keratin (from horn and hoof). 

This new use of the term albuminoid (literally albumin-like) 
must be distinguished from its now obsolete meaning, for in the 
past it was used as synonymous with “proteid,” and therefore at 
that time included albumin and its congeners as well as gelatin 
and allied substances. The term albuminoid thus replaces “pro- 
teoids,” which was at one time applied to “proteids” (now pro- 
teins) of the gelatin group. 

It is to be regretted that all scientists have not yet accepted 
the new meaning for the term albuminoid. Thus according 
to W. O. Atwater,t the American Association of Agricultural 
Colleges and Experiment Stations subdivide protein compounds 
into albuminoids, gelatinoids, and extractives. The first group 
(the albuminoids) includes white of egg, lean meat, casein, and 
wheat gluten; whereas the second group (the gelatinoids) in- 
cludes collagen and ossein from which gelatin is made. This 
confusion in terms is to be deprecated, and perhaps the best 
way to do would be to drop the term albuminoid entirely, sub- 


1Farmer’s Bulletin 142, U. S. Dept. of Agriculture, reprint January, 1921, 
p. 4. 
20 


POSITION OF GELATIN AMONG THE PROTEINS — 21 


stituting in its place the more descriptive term scleroprotein 
(proteins of the skin and skeleton). 

The position of the albuminoids or scleroproteins among the 
proteins may be seen from the following tabular classifications, 
which include also the products of hydrolysis. 


Classification of Proteins. 


The American Classification, adopted by the American 
Physiological Society and the American Society of Biological 
Chemists, is: 


I. StmpPLeE PROTEINS: 


Albumins—i.e. egg albumen; serum-albumin. Soluble in 
distilled water and in salt solutions; their acid and basic 
functions are almost equal, and they are salted out by 
saturation of their solutions with ammonium sulphate. 

Globulins—i.e. egg-globulin separated from egg white by 
dilution with distilled water; edestin from the seed of 
hemp (Cannabis Sativa). Insoluble in distilled water, 
but soluble in dilute solutions of strong acids or bases, 
or of inorganic salts. They are salted out by half satu- 
ration of their solutions with ammonium sulphate, or by 
complete saturation with magnesium sulphate. They are 
rather more acid than basic. 

Glutelins—i.e, glutenin from wheat; oryzenin from rice. In- 
soluble in distilled water or in dilute salt solutions, but 
soluble in dilute solutions of strong acids or bases. 

Prolamins—i.e. gliadin from wheat and rye; hordein from 
barley; zen from corn. Soluble in 70 to 90 per cent. 
alcohol, and in dilute solutions of strong acids or bases, 
but practically insoluble in distilled water. On hydrol- 
ysis they yield a large percentage of proline. | 

Protamines—i.e. salmine from salmon spermatazoa. The 
simplest natural proteins, usually found in combination. 
Predominantly basic substances soluble in water, and 
forming with acids compounds precipitated by alcohol. 
On hydrolysis they yield considerable diamino acids. 

Histones—i.e. the histone of hemoglobin which is there com- 
bined with the colored acid radicle hematin. Soluble in 


22 GLUE AND GELATIN 


dilute solutions of acids or of strong bases, but precipi- 
tated from acid solutions by ammonia. Less markedly 
basic than the protamines. 

Albuminoids  (Scleroproteins)—This large heterogeneous 
group is tentatively sub-divided as follows: 


(A)—CoLLAGENS OR JELLY-FORMING ALBUMINOIDS: 


(1) Collagen and gelatin; Dissolve more or less readily in 


from skins, bones, white 
fibrous connective tis- 
sue. 


drin; from permanent 
cartilages. 


boiling water, yielding solu- 
tions which gelatinize on cool- 
ing. Contain little or no sul- 
phur. 


Chondrigen and chondrin are 


really glycoproteins, but are 
mentioned here because they 
occur in glue and gelatin and 
in the materials from which 


(2) Chondrigen and sa 


they are made. 
(3) Isinglass; 


fishes. 
(4) Sericin (silk - gum) ; 
from silk. 


(B)—Fiproips: 


(1) Elastin; from elastic 
ligaments. 

(2) Fibroin; from silk and 
spiders’ webs. 


(C)—CHITINOIDS: 


(1) Chitin; from external 
shells of beetles, crabs 
or lobsters. 

(2) Chonchiohn; from 
shells of mussels and 
snails. 

_ (3) Spongin; from sponges. 


(D)—KezratTINs: 


(1) Keratin; from hoofs, | 


horns, feathers, hair, 
wool, etc. 
(2) Neurokeratin; from 
brains. 


from the 
swimming bladder of 


Undissolved by dilute acids, 


boiling water, or boiling very 
dilute alkali. Dissolved by 
stronger alkali. Contain no 
sulphur. Have high tensile 
strength. 


Insoluble in boiling water or in 


alkalis (spongin dissolves in 
concentrated alkali). Contain 
no sulphur. 


Insoluble in water, salt solu- 


tions, or dilute acids or al- 


kalis. Difficultly soluble in 
strong alkali. Contain sul- 
phur. 


POSITION OF GELATIN AMONG THE PROTEINS — 23 


II. ConsuGatep ProTeiIns:—Protein combined with a non-pro- 

tein radicle termed the prosthetic group. 

Nucleoproteins—from nuclei of cells. Compounds of a pro- 
tein (acting as a base) with one of the nucleic acids 
(substituted phosphoric acids containing carbohydrate 
and nitrogenous radicles). Insoluble in distilled water; 
soluble in dilute alkalis, such solutions being precipitated 
by weak acids such as acetic acid and carbon dioxide. 

Glycoproteins—here the prosthetic group is (1) an amino- 
carbohydrate; (2) a polysaccharide derivative of gluco- 
samin or its acetylated derivatives; (3) chondroitin-sul- 
phuric acid. 


There are three subdivisions: 


(1) Mucins—from mucous, snail-slime, etc., yield extremely 
viscous solutions from which the mucin is precipitated by 
acetic acid. 

(2) Mucoids—i.e. ovomucoid from egg white, are not as 
viscous in solution as mucins, and are not precipitated by 
‘acetic acid. 

(3) Chondroproteins—trom cartilage, amyloid tissue, etc., are 
insoluble in water. Their solutions in dilute alkali are 
precipitated by an excess of acetic acid or on neutraliza- 
tion with strong acids. The chondrovrtin-sulphuric acid 
they yield on hydrolysis is composed of one molecule of 
sulphuric acid with one molecule of chondroitin, itself a 
compound of glucosamin and glucuronic acid, which 
physically resembles gum arabic. 

ee reotcins—ie. casein. Predominantly acid proteins, 
yielding phosphoric acid on hydrolysis. 

Hemoglobins—i.e. hemoglobin, in which hematin, an iron- 
containing complex organic acid, is the prosthetic group, 
united with a histone-like predominantly basic protein 
globin. 

Lecithoproteins—here the prosthetic group is a phospholipin. 
It is questionable whether the phospholipin is chemically 
combined or is simply an adsorbed impurity. 


III. Derrivep PROTEINS: 


(A) Primary PROTEIN DERIVATIVES: 
(1) Proteans.—Insoluble products formed by the incipient 


24 


GLUE AND GELATIN 


action of water, very dilute acids or enzymes, e.g. myosan 
from myosin, edestan from edestin (Hawk). 

(2) Metaproteins—These result from further action of acid 
or alkali, are soluble in very weak acid or alkali but in- 
soluble in neutral fluids, e.g. acid metaprotein (acid al- 
buminate); alkali metaprotein (alkali albuminate). 

(3) Coagulated Proteins—lInsoluble products resulting from 
the action of heat or alcohol on protein solutions. 


(B) SeconpDARY PROTEIN DERIVATIVES: 


(1) Proteoses—Soluble in water; not coagulated by heat; pre- 
cipitated by saturation of their solutions with ammonium 
or zinc sulphate; e.g. protoproteose, deuteroproteose, 
gelatoses. 

(2) Peptones——These differ from proteoses in that they are 
not precipitated by saturating their solutions with am- 
monium sulphate, i.e. antipeptone, amphopeptone, gela- 
tones. 

(3) Peptides——These are really peptones whose structure is 
known; that is, the polypeptides of Fischer; e.g. glycyl- 
glycine, etc. | 

The classification of proteins adopted by the British Medical 


Association is as follows: 


Ae 


Lis 


SIMPLE PROTEINS: 


Protamines—e.g. salmine, clupeine. 
Histones—e.g. globin. 
Albumins—e.g. serum albumin. 
Globulins—e.g. ovoglobulin. 
Glutelins—e.g. glutelin. 

Alcohol soluble Proteins—e.g. zein. 
Scleroproteins—e.g. elastin. 
Phosphoproteins—e.g. casein. 


CONJUGATED PROTEINS: 


Glucoproteins—e.g. mucin. 
Nucleoproteins—e.g. nucleohistone. 
Chromoproteins—e.g. hemoglobin. 


POSITION OF GELATIN AMONG THE PROTEINS © 25 


III. Propucts or ProTtrin HypRo.Lysis: 


Infraproteins—i.e. acid or alkali albuminates formed by 
gently heating albumins in acid or alkali. Insoluble in 
distilled water; e.g. acid albuminate. 

Proteoses—e.g. protoproteose. 

Peptones—e.g. antipeptone. 

Polypeptides—e.g. dipeptides. 


-'T. Brailsford Robertson ? criticizes both the American and the 
British systems of classification as being based upon variations 
in physical behavior which do not necessarily correspond to dif-. 
ferences in chemical structure, whereas on the other hand there 
are many proteins or protein-like substances whose intermediate 
characteristics make their inclusion in any group a more or 
less arbitrary matter. His criticism is well-founded. 


Molecular Structure. 


This naturally raises the question as to the nature of the 
combination which holds together the various amino-acids in 
the molecules of different proteins. From the complexity of the 
amino-acids yielded on the drastic hydrolysis of gelatin and 
allied proteins (see table on p. 30), it is obvious that the 
combination is by no means as simple as ordinary two dimen- 
sional formule on paper would indicate. The classical work 
of Emil Fischer on the polypeptides shows that with these rela- 
tively simple (as compared with proteins) compounds, the link- 
age takes place according to the scheme first suggested by Hof- 
meister: | 

R’ Atif ¢ 


That is, the COOH and NH, groups of different molecules (the 
molecules may themselves be alike or different) combine with 
the elimination of water. 

But this relatively simple conception can not be carried over 
literally to proteins. In a trenchant criticism of Wolfgang 
Pauli’s ‘‘Kolloidchemie der Eiweisskorper,’ Wolfgang Ostwald ° 


2“Principles of Biochemistry,” p. 125. 
8 Kolloid Z. 27, 143 (1920). 


26 GLUE AND GELATIN 


quotes Emil Fischer as saying: “It can not with certainty be 
predicted whether a 20-polypeptid or a substance of like com- 
plexity of constitution would behave physico-chemically like 
albumin or not. It may happen sometimes, but not always. 
When things become so complicated the way they are consti- 
tuted is not so easily explained. They become so indefinite. .. . 
In the course of time I have built up ever larger molecules. ‘The 
colloid chemists would do well, like Perrin, to reverse this and 
‘from relatively large particles come down to molecules.” 

We must approach the subject without bias or fixed precon- 
ceived theories, and with minds flexible enough to fit all the 
facts of Nature, even though some be recently discovered facts. 
The complexity, frangibility, and even the variability of the 
so-called chemical elements, are established facts. The radio- 
active elements are spontaneously decomposing. Rutherford has 
shattered nitrogen by the impact of a particles shot out from 
radium at a speed of about 10,000 miles a second. By positive 
ray analysis Aston and others have demonstrated the existence 
of isotopes of many elements.*’ We have awaked to a realization 
of the fact that, just as there is no sharp line of demarcation 
between colloidal solution and true molecular solution or dis- 
persion, so too no sharp line can be drawn between physical and 
chemical attraction. 

Shght variations may mean much, e.g., the decimal in the 
atomic weight of hydrogen 1.008 represents electrons. The 
astronomer Herschel remarked that “the perfect observer will 
have his eyes, as it were, opened, that they may be struck at 
once with any occurrence which, according to received theories, 
ought not to happen, for these are the facts which serve as clues 
to new discoveries.” 

If the attractive forces existing between atoms (or atomic 
groups) were entirely satisfied or balanced by their chemical 
combination consequent upon the principal electronic attractive 
forces, or forces of primary valence as they are called, then every 
chemical compound would behave like a perfect gas so far as 
concerns the factor a in the equation of van der Waals. But in 
all chemical compounds there exist residual attractions or stray 

Thus there are three kinds of chlorine (atomic weights 35, 37 and 389 


respectively) and six kinds of mercury; consequently there are 18 different 
mereuric chlorides and 63 possible mercurous chlorides (Harkins). 


POSITION OF GELATIN AMONG THE PROTEINS — 27 


fields of force, which exert a controlling influence upon what we 
ordinarily call the physical properties of the compound—its state 
(gaseous, liquid or solid), its cohesion, solubility, melting point, 
freezing point, dielectric constant, conductivity for heat and 
electricity, etc. ‘These residual attractions are responsible for 
adhesion, adsorption, and the mechanical strength of materials, 
and their effective range of action (of the order of 10° cm.) is 
usually much less than the diameter of a molecule. 

Practically all molecules (and even all atoms) are polar and 
exhibit dissymmetry. They therefore tend to orient themselves 
so that their attractive forces may reach an equilibrium. This 
is particularly evident in cases of adsorption at surfaces where, 
as Langmuir ® observes, the molecules usually orient themselves 
in definite ways in the surface layer, since they are held to the 
surface by forces acting between it, and particular atoms or 
atomic groups in the adsorbed molecule.® 

Where the residual attractive forces reach an equilibrium, the 
molecules (or atoms) become more or less regularly distributed 
in the space lattice, and the compound (or element) is crystal- 
line, and usually shows its regularity of orientation when exam- 
ined by the X ray spectrometer of Bragg and Bragg.” In many 
cases, however, this tendency towards definite orientation is 
never realized. This is especially so where there are large and 
cumbersome molecules involved, as with the proteins, and even 
with metals and alloys where quick chilling tends to preserve 
the random or haphazard distribution of the atomic groups.® 
The experiments of P. Scherrer ® with the X-ray spectrometer 
show that colloidal gold particles too small to be seen even in 
the ultramicroscope, nevertheless show the same space lattice 
as macroscopic gold crystals. Old specimens of silicic acid and 
stannic acid gels exhibit well marked crystal interference figures 
in addition to the characteristics of amorphous substances, prob- 
ably representing substances on the point of crystallizing. But 
typical organic colloids such as gelatin, albumin, casein, cellu- 


5 J. Am. Chem. Soc., 40, 1363 (1918). 

®See also Harkins, Clark and Roberts, J. Am. Chem. Soc., 42, 706 (1920). 

7 Bragg and Bragg, ‘““X Rays and Crystal Structure,’ London, 1915. 

§See Jerome Alexander, “The Colloidal State in Metals and Alloys,” Trans. 
Am. Inst. Mining and Met. Eng., Vol. 69 (1921); presented at Columbus, O., 
meeting, October, 1920; Chem. Met. Eng., January, 1922. 

® Nachr. Ges. Wiss. Géttingen, 96-100 (1918). 


28 GLUE AND GELATIN 


lose, and starch appear to be amorphous. The colloid particles, 
therefore, probably consist of large individual molecules or of 
groups of irregularly oriented molecules. 

Considering the information at present available, it would 
appear that the forces binding the relatively simpler molecular 
units into a “molecule” of gelatin are largely what have here- 
tofore been considered “physical” forces. Perhaps the difficul- 
ties of nomenclature may be to some extent avoided if we adopt 
the suggestion of P. EH. Wells.?° 

(1) Electronic forces—maintain positive nucleus and negative 
or valence electrons in equilibrium as a single system. 

(2) Atomic forces—maintain two or more atoms in equi- 
librium as a single system. 

(3) Molecular forces—maintain two or more molecules in 
equilibrium as a single system. 

(4) Molar forces—maintain two or more masses in equilibrium 
as a single system. 

“Hach group of forces may be regarded as the residual fields 
of force remaining unsaturated in the smaller systems con- 
stituting the components of the system under consideration... . 
Molecular systems have lost so much of their discreteness that 
combinations of molecules do not follow the laws of definite 
and multiple proportions. In such phenomena as molecular 
association and surface structure, the discreteness of atomic con- 
stitution begins to give way to statistical continuity. More- 
_ over, even in these phenomena, the forces are relatively so weak 
that molecules are not usually regarded as permanently grouped 
together.” (Wells, loc. cit.) 

What are generally called “chemical forces” are the atomic 
forces in the above classification, whereas what are generally 
called ‘physical forces” are the molecular forces therein men- 
tioned. A careful consideration of the experimental facts of 
physical chemistry, i.e. ionization, hydrolysis, adsorption, differ- 
ential diffusion, association, and dissociation, clearly show that 
it is just as impossible to draw a sharp line of demarcation be- 
tween physical and chemical compounds as it is to separate by 
fixed lines the several primary colors of the spectrum. 

It must not be supposed that this difficulty of definition is 
a new matter. Thus in 1884 in discussing a paper on adsorp- 

10 J, Wash. Acad. Sct. 9, 361 (1919). 


POSITION OF GELATIN AMONG THE PROTEINS 29 


tion by John Uri Lloyd," Prof. Prescott. said: “It has been 
asked whether we should refer Prof. Lloyd’s results to chemical 
action within molecules or to those forces which may be classed 
under physical action. Now we do not know a great deal about 
these modes of force, or their essential nature; but I think we 
know this much, that there is no sharp line of demarcation 
between chemical action within the molecules, and the physical 
action between the molecules. They grade off into each other. 
We must look for the interference of adhesion in a great many 
operations called chemical.” 

The same view has been voiced by many others, and recently 
by P. P. von Weimarn,” although but a short while before, he 
had regarded the condensation gas — liquid — solid as a chemi- 
cal phenomenon. 

In conclusion it may be said that, while the formation of 
amino-acids and even of some of the more complicated poly- 
peptides is controlled by the action of atomic forces, with in- 
creasing complexity of constitution, molecular forces appear 
and eventually predominate. The view held by many chemists 
that all attraction must follow the same simple laws that govern 
the formation of simpler compounds, must be abandoned; for, 
as Poincaré remarked, Nature is not as simple as all that. 

Once we realize the fact that gelatin has a coarser physical, 
as well as a finer chemical structure, we may be able to under- 
stand its hydrolysis and degradation without having recourse 
to ingenious but weird purely chemical explanations or formulas 
which have no counterpart in the actual facts, although in some 
cases they may not be disproved by the present experimental 
evidence. 


11See J. Am. Pharmaceutical Assoc. for October, 1916. 
12 Kolloid Z. 28, 97 (1921). 


Chapter 3. 


The Chemistry, Physical Chemistry, and Colloidal 
Chemistry of Gelatin and Glue. 


Since the chemistry of gelatin is inseparably bound up with 
its physical chemistry and its behavior as a colloid, confusion 
only can result if an attempt be made to discuss these aspects 
separately... We have already considered the position of gelatin 
among the proteins, and the general nature of the forces hold- 
ing its constituent atomic groups together (Chapter 2). Let 
us now consider the more intimate structure of the gelatin 
“molecule.” 


Chemical Structure. 


The most important evidence we have regarding the chemical 
nature of gelatin, is given by the products of its hydrolysis. 
Skraup and von Biehler+ found that on hydrolysis with hydro- 
chloric acid, gelatin yields the following substances, all of which 
had been found by previous workers. 

Glycine—a-amino-acetic acid. CH,(NH,).COOH 
LLysine—oa-e-diamino-caproic acid. 

CH,.NH,(CH,),.CH(NH,) COOH 
Alanine—a-amino-propionic acid. CH,.CH(NH,).COOH 
Phenylalanine—f-phenyl-a-amino-propionic acid. 

C,H,.CH., CHiN.) COC 
Leucine—a-amino-isocaproic acid. 

CH, .({CH,),.CH(NH.) COGS 
Aspartic acid—amino-succinic acid. . 

COOH.CH,.CH(NH,) .COOH 
Glutamic acid—o-amino-glutaric acid. 

COOH.CH,.CH,CH(NH,) .COOH 
Histidine—f-imino-azole-a-amino-propionic acid. 

N — C.CH,.CH(NH,) .COOH 


1 Monatshefte fiir Chemie 30, 476 (1909). 
30 


THE CHEMISTRY OF GELATIN AND GLUE 31 


Arginine—o-amino-$-guanino-n-valeric acid. 
NH:C(NH,) .NH(CH,),.CH(NH,) .COOH 
Proline—a-pyrrolidine-carboxylic acid. H,C — CH, 


ie 

H,C — C.COOH 
bo eae 
NH 


Oxyproline—f-oxy-a-pyrrolidine-carboxylic acid. 
H,C — CH.OH 


Ene. 
H,C CH.COOH 
~~ Ti 
NH 


Whether these amino-acids exist in the gelatin “molecule” as 
such, or are formed from the disintegration of larger molecules, 
cannot with certainty be decided at present. 

To see how the degradation products of gelatin compare with 
those of other albuminoids or scleroproteins, there is given the 
following table prepared mainly from “The Chemical Constitu- 
tion of the Proteins,” by R. H. Alders Plimmer: 


a o “ 
33 s 3 ae 
<5 Stale a2 0 fe 
OM a Bly, a8 
e b a alae: S 2 ar 
a ee AN eens Sess 
peer 8 eee 
> cs} eo} s¥ sea Se 
poets as he Bo eee Se 
poss | 8200 eS 0lC(USeOUCUES ge" 2 Sey 
Bamecde So SSCS. Se BEG Sa 
Seces 85 sk 82 se SSar. 825 
Soe cam os ca. a ne Base MES 
Glycine ..... 25.5 16.5 1925 124 360 S025 208 0.4 
Alanine ..... 8.7 0.8 3.0 Q. 21.0 23.4 6.6 1.2 
OU are 0.0 1.0 — _ 0.0 — 1.0 Bik 
Leucine yaa 2.1 6.75 9.2 1.5 8 214 18.3 
Isoleucine ... 0.0 0.0 — — — — — 
Phenylalanine 1.4 0.4 — 1.0 1.5 — 3.9 3.0 
Tyrosine .... 0.01 0.0 — — 10.5 8.2 0.4 46 
Serine ...... 0.4 0.4 — — 16 — — 0.7 
Cystine ..... — — — — — — — 6.8 
Proline. ..... 9.5 5.2 6.25 104 + 3.7 1.7 3.6 
Oxyproline .. 14. 3.0 6.4 3.0 — — — — 
Aspartic Acid 3.4 06 — 12 + — — 2.5 
Glutamic Acid 5.8 0.9 1.75 168 0.0 11.7 0.8 3.0 
Lysine ...... 59 28 — 6.0 <b 5.24 — — 
Arginine .... 8.2 76 — 9.3 1.0 — 03 2.3 
Histidine ... 09 0.4 — 0.4 + — — — 
Ammonia ... 04 0.4 — 0.4 — 12 — —- 
Total per cent. 91.31 42.1 434 70.7 73.1 90.44 61.9 52.1 


32 | GLUE AND GELATIN 


Y. Okuda? hydrolyzed the gelatin from shark skin with 
hydrochloric acid, baryta, and sulphuric acid, and found that 
the fish gelatin gave somewhat more monoamino acid, and much 
more glycocoll, alanine, leucine, phenylalanine, glutamic and 
aspartic acids than does bone gelatin. The diamino acid con- 
tent was about the same in both gelatins, but the proline and 
serine content of fish gelatin was low, perhaps because of experi- 
mental error. 

Many theories have been advanced to explain the amphoteric 
nature of the proteins, i.e. their ability to combine with either 
acids or bases. H. Bechhold® gives the following abstract of 
the theory, proposed by G. Bredig and extended by W. Pauli ** 
as to the amphoteric nature of albumin; and as these apply 
equally to gelatin, they are given here at length. 

“Let us think of albumin as being built according to the struc- 
ture of a cyclic* ammonium salt: 


; NH, 
Riceae 
COO 


in which R represents a complicated organic complex, and the 
absorption of water follows according to the scheme: 


NH, NH,OH 
COO COOH 


“This is an amphoteric electrolyte which unites with bases and 
acids, which splits off H as well as OH ions, and in which the 
K, (acid dissociation) > Kp (basic dissociation); in these 


words, it behaves like a very weak acid. Pure albumin consists 


2J. Coll. Agric. Tokyo 5, 355 (1916). 

3 “Colloids in Biology and Medicine,” p. 154. 

3a In a recent paper (Kolloid Z. 28, 49 [1921]) entitled “Der Allgemeine 
Bauplan der Kolloide,’ Pauli expresses the view that with albumins the amino- 
acids chemically combined with each other, form neutral particles which receive 
‘their chargé from one or a few ionizing amino-acids. Wo. Ostwald, J. Loeb, 
and many others, including the author, do not agree with all of these views 
of Pauli, which are given in some detail as they represent one of the purely 
“chemical” views of the behavior of gelatin. 

‘This is what Winkelblech terms an internal salt, whence the anhydride 


NH 
Rex | is formed by the elimination of water. 
Co 


THE CHEMISTRY OF GELATIN AND GLUE 33 


principally of electrically neutral particles, but forms acid and 
alkali salts which are strongly ionized. 
There exist 


NH,OH NH,Cl NH,OH 
Bat a R< 
COOH COOH COONa 
neutral albumin —_ acid albumin alkali albumin 


“That the albumin ions are responsible for the great internal 
friction is to be assumed from the investigations of E. Laqueur 
and O. Sackur on alkali-caseinates. The cause of this phe- 
nomenon is found in the strong hydration (water fixation, swell- 
ing) of the albumin ions.° According to Wo. Pauli and M. 
Samec the existence of polyvalent ions must be assumed in the 
case of acid and alkali albumin. Even assuming the smallest 
values for the molecular weight gf albumin, the quantities of 
acid or alkali found are so large that they indicate the fixation 
of several acid or alkali molecules. This offers a further ex- | 
planation of the marked increase in hydration produced by acids | 
and alkalis. The stability of an albumin solution and its pre- | 
cipitability, e.g. by alcohol, are directly proportional to the / 
number of albumin zons it contains. The circumstances here are | 
quite analogous to those with crystalloids. Ions tend to go into | 
solution and to form hydrates; the saturation concentration of | 
neutral particles is always less than that of ions. i 

“In this way we may explain the properties of strongly ionized 
pure acid and alkali albumin as contrasted with the slightly dis- 
sociated neutral albumin. 7 

“How does this theory agree with the effect of neutral salts? 
Wo. Pauli explains it in the following way: 


NH,Cl NH,Cl 
Ki + NaNO,2R< + HNO, 
COOH COONa 
acid albumin neutral Na salt of 
salt acid albumin 


In this way was explained not only the increased number of free | 
H ions, which he demonstrated, but also the marked diminution | 
in the internal friction; because an amphoteric salt, in which | 


5 That the hydration rather than the size of the protein ions is the cause of 
their non-filterable character, is the view expressed by T. B. Robertson (‘‘The 
Physical Chemistry of the Proteins,” p. 148, footnote), J. A. 


? 


34 GLUE AND GELATIN 


both anions and cations tend to ionize about equally, is but 
slightly dissociated. 

“The action of neutral salts on alkali albumin is different; it 
follows the following scheme: 


2 
ae Ri +kCl2R< + H,O 
ees COONa COONa_~ water 
© us) alkali albumin neutral complex 
si salt albumin 
yan salt 


ai ow “Accordingly, a complex albumin salt is formed to which a less 


ce 


amount of ionization may be ascribed than to alkali albumin. 
The action of salts of the alkaline earths follows this scheme: 





NH;OH se Gn" NH,NaNO, 
R< +3-NO, 2 R< Ca foe 
COONa Coos 


The replacement of the alkali ion in the hydroxyl of the amino 
group results in a weakly ionized complex salt. The effect on 
albumin of organic bases, which are often highly toxic, and of 
amphoteric electrolytes, have also been studied by H. Handovsky, 
and the results agree with the above scheme. 

“The conditions governing the action of neutral salts upon acid 
albumin are not sufficiently understood to warrant proposing a 
simple scheme.” 

In a footnote Bechhold remarks that it should not be assumed 
that only free terminal NH, groups are to be considered, since 
the work of Blasel and Matula on deaminized gelatin make it 
probable that interior NH, groups are involved. 

The method of Blasel and Matula * for deaminizing gelatin is 
as follows: 

To a solution of 200 grams of the purest commercial gelatin 
in 1 liter of warm water, is added 200 grams of sodium nitrite 
also dissolved in 1 liter of water. After cooling 140 grams of - 
glacial acetic acid is carefully added, and after standing 12 hours, 
the mixture is heated for two hours on a water-bath. The 
deaminized gelatin is then salted out by saturation with am- 
monium sulphate and purified by prolonged dialysis (2 weeks) 
against running distilled water. 

5a Biochem, Z. 58, 417 (1914). 


THE CHEMISTRY OF GELATIN AND GLUE 35 


The deaminized gelatin, although its free amino groups are 
all destroyed, still has almost the same acid-combining capacity 
as ordinary gelatin. This certainly indicates that something 
other than the chemical attraction of the free NH, groups is 
responsible for acid fixation. 

T. B. Robertson ° believes that the — CONH — groups within 
the molecule are responsible for the acid-and-base-combining 
capacity of the proteins. The — CONH — group may exist in 


the keto form, — CO — NH —, or in the enol form, = = N-—, 
On 

but neither analytic or synthetic methods are able to distinguish 

between the two forms. Robertson thinks that the enol form is 

most probable, since it can attach either acids or bases. 

D. Jordan Lloyd’ thinks that “a more probable explanation 
seems to be that under the action of acids gelatin goes into the 
keto-form, and under the action of bases to the enol-form. This 
view would conform with the observation that the free acid from 
sodium gelatinate differs in properties from the free base of gela- 
tin hydrochloride. It can also be harmonized with Dakin’s 
theory ® that the non-terminal groups in proteins go from the 
keto-form (1) to the enol-form (2) © 


(1) NH.CO— (2) NH.CO — 
es ye 
R.C—C—NH.CHR.COOH R.C=C—NH.CHR.COOH 
ae | 7 
1s een 6) OH | 
with loss of optical activity under the action of bases at low 
temperatures.” 


Wintgren and Kriiger® believe that since proteins have more 
than one NH, group, it is only in dilute solutions that we find 
the type 

[GE] NH,* + H,Os [GE] NH,OH-+ Ht 
corresponding to 
NH, + H,O = NH,OH + H* 
8 “Physical Chemistry of the Proteins,’ p. 24; ‘Principles of Biochemistry,” 
. 156. 
i 7 Biochem. J. 14, 154 (1920). 


8 J. Biol. Chem. 18, 357 (1913). 
® Kolloid Z. 28, 81 (1921). 


36 GLUE AND GELATIN 


In higher acid concentration several amino groups take part in 
salt formation, so that ‘proteins have not one but several dis- 
sociation constants whose values decrease at. varying speeds.” 


Loeb’s Theory of Colloidal Behavior. 


Jacques Loeb 7° concludes from the results of a series of care- 
ful and ingenious experiments that it is the H-ion concentration 
of protein solutions which controls their behavior. “Proteins 
exist in three states, defined by their hydrogen ion concentration, 
namely (a) as non-iongenic or isoelectric protein, (b) metal pro- 
teinate (e.g. Na or Ca proteinate), and. (c) protein-acid salts 
(e.g. protein chloride, protein sulphate, etc.). We will use gela- 
tin as an illustration. At one definite hydrogen ion concentra- 
tion, namely 10%’ N, or in Sorensen’s logarithmic symbol at 
ec Yn gelatin can combine practically with neither anion 


nor cation of an electrolyte. At p,,>4.7 it can combine only 


with cations (forming metal gelatinate, e.g. sodium gelatinate), 
at p.,<4.7 it combines with anions (forming gelatin chloride, 


etc.) .” 

Loeb then describes experiments with powdered gelatin swollen 
in ice-cold water, showing that only those gelatins having 
Py, >4.7 can fix Ag from AgNO, or Ni from NiCl,; and only 


those having p,,<4.7 can fix Fe(CN),. “In this way it can be 
shown,” says Loeb, “that when the p,, is >4.7 gelatin can com- 
bine only with cations; when p,, is <4.7 it can combine only 
with anions, while at p,, 4.7 (the isoelectric point) 1t can com- 


bine neither with anion or cation. The idea that both ions 
influence a protein simultaneously is no longer tenable. 

“Tt follows also that a protein solution is not adequately de- 
fined by its concentration of protein but that the hydrogen ion 
concentration must also be known since each protein occurs in 
three different forms—possibly isomers according to its hydrogen 
ion concentration.” , 

Loeb claims that the direct chemical union of acids with 


10 Science, N. S., Vol. 52, p. 449 (1920). : 

11 An H+ concentration of 2 x 10-5 is expressed according to Sdrensen’s nota- 
tion as follows: p y (the H-ion concentration) =—log (2X 10>) =— (0.3-5) 
Sb (le ee) 6 


THE CHEMISTRY OF GELATIN AND GLUE 37 


gelatin is demonstrated by his experiments, which show that 
3 times as many cc. of 0.1 N H,PO, are required to bring 100 cc. 
of 1 per cent. gelatin solution to a given p,,, as are required in 


the case of HCl or HNO,, twice the number of cc. of 0.1 N 
oxalic acid, and the same number of cc. of 0.1 N H,SO, (... in 
a strong dibasic acid, like H,SO,, both hydrogen ions are held 
with a sufficiently small force to be easily removed”). Bases 
show analogous results, and in conclusion Loeb makes the rather 
sweeping assertion—“The behavior of the proteins, therefore, 
contradicts the idea that the chemistry of colloids differs from 
the chemistry of crystalloids.”’ 

If all molecular forces are to be regarded as ‘“‘chemical,”’ then 
Loeb’s case is proved at the outset by definition. 

W. D. Bancroft ?? takes Loeb to task for drawing general con- 
clusions on the basis of experiments made with dilute solutions. 
Bancroft says: “Under the conditions of the experiments Loeb 
found that on the acid side of the isoelectric point only anions 
of neutral salts are taken up and the alkaline side of the iso- 
electric point only cations. Since the Hofmeister series calls for 
an effect due to both ions of a neutral salt on the swelling of 
gelatin, Loeb concludes that the Hofmeister series is a delusion 
and a snare. This does not follow at all. Loeb is working at 
such extreme dilutions that the specific effects of all ions but 
hydrogen and hydroxy! ions are practically negligible. In acid 
solutions only anions are taken up and in alkaline solutions only 
cations. Loeb recognizes the specific effect of iodine ions over 
chlorine ions in causing the liquefaction of gelatin, but he con- 
siders that liquefaction stands-in no necessary relation to swell- 
ing, an assumption which will be shared by few. With higher 
concentrations Loeb will undoubtedly get entirely different re- 
sults. His conclusions as to the existence of definite compounds 
depend on the assumption that he is dealing with true solutions 
and will fall with that assumption.” 

In reply to Bancroft, Loeb ** points out that “salt solutions 
up to grammolecular concentration were used without any indi- 
cation of validity of the Hofmeister series being found. Ban- 
croft will surely not maintain that solutions of neutral salts up 
to molecular concentration are so dilute that the effects of all 


2 “Applied Colloid Chemistry,” p. 255 (1920). 
33 “Proteins,” p. 110. 


38 GLUE AND GELATIN 


ions except the hydrogen and hydroxyl ions are practically 
negligible. 

“The writer’s (Loeb’s) statement that the liquefaction of solid 
gelatin stands in no necessary relation to swelling is correct, 
since higher concentrations of acids or of salts like CaCl, 
diminish the swelling of gelatin while they increase its solubility. 
This is due to the fact that swelling and solution of gelatin in 
the presence of acid are functions of different variables, swelling 
in acid depending on the Donnan equilibrium, while the solution 
of gelatin depends on the same forces which are responsible for 
the solution of ordinary crystalloids in water (probably second- 
ary valency forces).” 

Because of the facts pointed out in Chapter 4, it seems to the 
writer that Bancroft’s criticism is well founded. The Donnan 
equilibrium is based on the assumption that there is complete 
ionization of the colloid salt (“gelatin chloride”) and the crystal- 
loid (HCl). As concentrations increase, the degree of dissociation 
and ionization diminish. Another assumption is that the jelly 
cation of “gelatin chloride” is not diffusible; but as D. Jordan 
Lloyd points out (loc. cit., p. 164) the ‘colloidal ion” is dif- 
fusible to some extent and must exert an osmotic pressure. 

C. R. Smith +* has shown that traces of impurities, especially 
Ca salts, exercise a potent influence on the water-absorbing ca- 
pacity of gelatin, and that this fact may vitiate many of Loeb’s 
conclusions which were based on experiments made with ash- 
containing gelatin. Smith points out that the increased swell- 
ing observed by Loeb in gelatin treated with sodium chlorid, etc. 
(all excess being washed out), 1s due to the fact that “these 
’ electrolytes remove the repressing lime salts and leave a gelatin 
combined with sodium cations. . . . It is not surprising that cal- 
cium, magnesium, strontium, barium chloride, or magnesium 
sulfate produce no increased swelling, for they do not remove 
the ash, and they also leave combined bivalent cations which 
do not increase swelling as much as univalent cations. Loeb 
continued to treat gelatin with various salts, under the impres- 
sion that they were reacting with the gelatin. Only when using 
oxalates does he mention the formation of 'a white precipitate 
(obviously from the lime). He (January 20, 1919) ascribes the 
Increase in osmotic pressure of gelatin to an increase in the 

4 J, Am. Chem. Soc. 43, 1850 (1921). 


THE CHEMISTRY OF GELATIN AND GLUE 39 


number of particles, ionization not considered, but later stated 
that free hydrobromic acid represses the ionization of gelatin 
bromide and again that the physical properties of gelatin are 
dependent only on the number of gelatin bromide molecules 
formed. . . . Loeb’s figures for osmotic pressure obtained on in- 
completely purified gelatin are from 25 to 50 per cent.’ too low. 
The results of this paper, however, confirm many of his con- 
clusions.” 

Loeb in his reply to Smith* publishes the analysis by Dr. 
Hitchcock of his laboratory, of two random samples of the kind 
of gelatin he used. (Cooper’s gelatin purified by treatment with 
0.0078 M acetic acid and washing with distilled water of p,, 


a little above 5.0). The results showed 0.001 per cent. of ash, 
with qualitive tests for Fe, Ca, and PO,, but negative tests for 
Cl and S8O,. From this Loeb concludes that the ash content of 
the gelatin he had been using for experiments on swelling, os- 
motic pressure, and viscosity, ‘might have been about 1 mg.” 
(per gram). “It was shown by the writer’s (Loeb’s) experi- 
ments that that amount of ash (which equals roughly a 
0.000033 M solution of tricalcium phosphate) has no influence 
on the physical properties of the proteins, such as osmotic pres- 
sure swelling, viscosity, or potential difference.” 

It would be more convincing if Loeb had determined the ash 
of the particular specimens of gelatin that he actually used. He 
says that Smith’s criticism, that his results on the osmotic pres- 
sure and swelling of gelatin are vitiated by the use of ash-con- 
taining gelatin, does not apply to his more recent papers pub- 
lished during the last three years. Loeb states that the correct 
values for the osmotic pressure (of solutions containing 1 g. of. 
originally isoelectric gelatin in 100 cc.) are given in the May 
(1921) number of the Journal of General Physiology. ‘Former 
values were lower since the solutions contained less than 1 g. in 
100 cc., usually 0.8 g., as was pointed out in a paper published 
in January, 1921, in the same Journal.” 


Ash-free Gelatin. 
C. R. Smith applied for a public service patent (Serial No. 


390,253 dated June 19, 1920), and points out+** that J. Loeb 


15 J, Am. Chem. Soc. 44, 214 (1922). 
wa J, Am. Leather Chemists’ Assoc., Oct., 1922. 


40 GLUE AND GELATIN 


erred in‘ crediting Miss Field as the discoverer of ash-free gelatin. 
Ash-free gelatin looks like the ordinary kind, but a one per cent. 
solution soon becomes turbid. 

C. R. Smith’s method of preparing ash-free volun is here 
epitomized: 

Gelatin of the highest jelly strength (maximum mutarotation 
ratio 2.2), ground to about 16 mesh, is washed on a filter with 
cold (0° to 10°) 10 per cent. sodium chloride solution containing 
5 ce. of concentrated hydrochloric acid per liter, until the wash- 
ings are free from lime. The acid is washed out with cold 1 per 
cent. sodium chloride solution, and then gradually weaker salt 
solutions are used. Distilled or conductivity water is finally 
-used to wash the gelatin until the washings show no chlorine. 
After dehydrating with cold 90 per cent. alcohol, the gelatin is 
then dried. 

“When powdered gelatin is washed with cold water alone, the 
readily diffusible calcium salts soon pass away until further 
washing becomes ineffective. If it is now washed with a solu- 
tion of sodium chloride, ammonium chloride, potassium bromide, 
or presumably any uni-univalent electrolyte, dialysis of the re- 
maining lime salts takes place immediately, probably because 
certain slowly diffusible, possibly colloidal, salts of calcium react 
with them to form readily diffusible salts. If the added electro- 
lyte is now washed out, any alkali combined with the gelatin is 
almost invariably left. Using sodium chloride, sodium carbonate 
is found in the ash. In order to insure the removal of this alkali 
as well as iron, heavy metals, etc., acidulated salt solution must 
be used. The removal of all calcium salts can be accomplished 
in an hour, but the removal of the hydrochloric acid requires 
several hours and the use of dilute salt solution until the remain- 
ing acid can be removed by water alone without excessive swell- 
ing. It is almost impossible to wash gelatin swollen to 40 or 
50 volumes. As the last traces of acid are being removed, the 
gelatin (at 15°) shrinks to particles swollen to about 7 volumes. 
The removal of the last traces of acid is probably facilitated by 
the fact that the isoelectric point of gelatin is on the acid side *6 

snare 


Gs free gelatin thus obtained when incinerated leaves no ash 


16 Michaelis, ‘‘Die Wasserstoffion Konzentration” ; Patten and Kellems, J. Biol. 
Chem. 42, 363 (1920). 


THE CHEMISTRY OF GELATIN AND GLUE 41 


other than the traces of sand when the original glue or gelatin 
contains such. When ashed with pure sodium carbonate, chlo- 
rides, sulphates, or phosphates cannot be detected. 

“Ash-free gelatin swells in water at 15° to about 7 or 8 vol- 
umes. If such a gelatin be melted and cooled, a clear, stable 
jelly is produced. If, however, a weaker jelly be prepared, 
syneresis takes place, with the production of a cloudy jelly. A 
0.5 per cent. jelly will flocculate into jelly particles (probably 
swollen to 7 volumes) and can be filtered off completely from 
the extruded water, which shows no trace of gelatin. 

‘““Ash-free gelatin forms sols or gels with a minimum tendency 
to remain dispersed. It is readily precipitated by alcohol with- 
out the presence of electrolytes. Traces of acids or alkalis 
increase the osmotic pressure and prevent its precipitation by 
alcohol. Gelatin thus peptized by traces of alkalis or acids 
in the presence of a large percentage of alcohol exhibits a marked 
resemblance to the metal suspensoids. Traces of electrolytes, 
for example those present in a drop of tap water, cause immedi- 
ate precipitation. (This indicates the protective action of hy- 
drolysis products or “impurities” usually present in gelatin. 
J. A.) Bivalent and trivalent ions are most effective in bringing 
about precipitation. 

“Ultimate analysis of this gelatin gave the following results:*7 


Carbon Hydrogen Nitrogen Oxygen 
50.47 6.75 17.53 25.25 
50.56 6.87 17.53 25.04 
50.52 6.81 i7cbe 25.15 


“Moisture was determined by drying at room temperature 
over sulphuric acid to constant weight for several weeks. Heat- 
ing at 100° caused no further loss in weight. Moisture correc- 
- tion was applied to all figures. The carbon content was from 

-0.5 to 1.1 per cent. higher than that in published analyses made 
on ash-containing material, probably because the latter retained 
carbon or carbon dioxide which was not considered.” 

Sheppard, Elliot and Benedict 1 report that gelatin free from 
ash and hydrolytic products may be prepared by electrolyzing 
a 5 per cent. solution of commercial gelatin in a cell of electro- 
filtros for three to four weeks, the salts passing through the cell 


7 Carbon and hydrogen determinations were made by Dr. D. H. Brauns. 
“aS. EH. Sheppard, Felix A. Elliot, and Miss A. J. Benedict, Science 46, 
550 (1922). 


42 GLUE AND GELATIN 


into the electrode chambers, and reducing the ash to about 0.10 
per cent. This partially de-ashed solution is then precipitated 
by acetone, thus removing the hydrolysis products and _ still 
further reducing the ash to about 0.01 per cent. The gelatin 
thus purified is dissolved in conductivity water, chilled in sheets, 
and dried. They recommend it for all research work on gelatin, 
as well as for providing culture media which can be brought to 
any particular reaction with complete knowledge of the salts 
present. 


Fischer’s Views. 


Martin H. Fischer 8 says: “The measurable hydrogen and 
hydroxyl ion contents of different protein-water systems upon 
which such emphasis has been laid for the explanation of their 
stability are only observable in relatively dilute systems; the 
ion contents are not inherent to, or necessary for, the stabiliza- 
tion; they are accidental accompaniments incident to the solu- 
tion of some of the acidic and basic proteins in the excess of 
water and their hydrolysis with the production secondarily of 
an overplus of hydrogen or hydroxyl ions.” 1° 

While recognizing the formation of chemical compounds in 
protein-acid and protein-alkali systems, Fischer says (loc. cit.): 
“How inadequate for the understanding of the colloid-chemical 
behavior of such systems are the overplayed ‘stoichiometrical,’ 
‘chemical,’. ‘electrical,’ hydrogen and hydroxyl ion notions, 
usually called upon to explain in some exclusive fashion all the 
changes observed, must be self-evident. 

“Stoichiometrical views cover only those parts of the whole 
problem which have to do with the quantities produced of dif- 
ferently hydrateable or soluble compounds; ‘chemical’ notions 
are no more adequate for the explanation of the problem than 
they are, at present, for the understanding of the whole problem 
of solution; electrical and ionic notions are hardly of service 
when it is remembered that the most stabile of these hydrated 
colloid systems are such as are composed of chemacally produced, 


18 “Soaps and Proteins,” p. 214. 

This recalls the behavior of ferric chloride, dilute aqueous solutions of 
which slowly hydrolyze and deposit Fe(OH); from a weak solution of HCl. 
By pouring a few drops of strong ferric chloride solution into boiling water, the 
decomposition takes place instantaneously, and is evidenced by the intense 
color of the colloidal Fe(OH)s3, which however soon precipitates. J. A. 


THE CHEMISTRY OF GELATIN AND GLUE 43 


really neutral compounds of protein with base or acid, provided 
only that not more water is present in the system than can be 
absorbed by the hydration capacities of the protein derivatives. 
Yet these colloid systems contain no quantities of either hydro- 
gen or hydroxyl ions measurable by ordinary laboratory means.” 

Taking up the case of gelatin specifically, Fischer *° says: 

“Dry gelatin absorbs water (to yield the system water-dis- 
solved-in-gelatin) and has a limited solubility in water (to yield 
the system gelatin-dissolved-in-water). Between these extremes 
and depending merely upon the relative amounts of gelatin and 
water present there lie the systems gelatin-solution dispersed in 
hydrated-gelatin (gel) or, with more water, hydrated-gelatin 
dispersed in gelatin-solution (sol). 

“What is the action of alkalis (or acids) upon these systems? 

“Under variously worded headings this problem has received 
much study. The effects of alkalis (and acids) upon the lower- 
most of the four systems may be found under the caption ‘swell- 
ing’ of gelatin in the presence of acids and alkalis; *! their effects 
upon the system gelatin-solution-in-hydrated-gelatin under the 
heading liquefaction and ‘solution’ of gelatin; 7? their effects upon 
the system hydrated-gelatin-in-gelatin-solution under studies in 
viscosity ; 7° their effects upon the system true solution of gelatin- 
in-water as studies on the ‘solubility’ of gelatin.2* What is the 
relationship between all these? 

“Tt is well to begin by inquiring into the relationship between 
the swelling of ‘soluble’ ‘neutral’ protein and its ‘solution.’ The 
notion that solution is but a continuation of swelling persists 
to this day.2° Investigation ?° of the problem, however, has 


20 Loc. cit., p. 218 et. seq. 

21See for example K. Spiro, Hofmeister’s Beitrdge 5, 276 (1904); Wolfgang 
Ostwald, Pfliiger’s Arch. 108, 563 (1905); M. H. Fischer, ‘‘Edema and Ne- 
phritis,” 3d ed., p. 75, New York, 1920, where references to earlier studies may 
be found. 

2M. H. Fischer, Science 42, 223 (1915) ; Kolloid Z. 17, 1 (1915). 

23 See for example the work of Hofmeister, Pauli, Hardy, von Schroeder, 
Handovsky, Schorr, ete., on the viscosity of liquid proteins (‘‘sols’’). 

24M. H. Fischer, ‘‘Hdema and Nephritis,”’ 3d ed., p. 518. As of similar impart 
but upon other proteins may be cited some studies on wheat gluten. T. B. 
Wood and W. B. Hardy (Proc. Roy. Soc. London, Series B, 81, 38 (1908), the 
influence of acids, while F. W. Upson and J. W. Calvin (J. Am. Chem. Soc. 87, 
1295 [1915]) studied its swelling under similar circumstances. 

2>See for example Wolfgang Pauli, ‘‘Kolloidchemie der Hiweiss Korper,’’ p. 
63, Dresden (1920). 

26M. H. Fischer, Science 42, 228 (1915) ; Kolloid Z. 17, 1 (1915). 


44 . GLUE AND GELATIN 


shown that this is not the case. The matter is easily proved by 
working with gelatin at concentrations and temperatures near 
its gelation or melting point. Since alkalis and acids increase 
hydration (increase swelling) the addition of these substances 
to a barely liquid gelatin-water mixture ought to stiffen it. As 
a matter of fact just the reverse occurs. By working with a 
stiff gelatin, a previously solid mixture is made to liquefy upon 
the addition of these substances. 

“The phenomena of swelling (hydration) and of ‘solution’ ** 
in such soluble protein gels as gelatin, while frequently assoct- 
ated, are therefore essentially different. Swelling 1s best under- 
stood as a change whereby the protein enters into phystco-chemi- 
cal combination with more of the solvent (water), as a change 
in the direction of greater solubility of the solvent in the pro- 
tein; ‘solution’ is best conceived of as a change in the direction 
of greater solubility (an increased degree of dispersion) of the 
colloid in the solvent. . . . (p. 220) under the influence of added 
alkali or acid the ‘neutral’ gelatin is converted into a basic 
gelatinate or gelatin chloride. These compounds, at the same 
concentration, are more soluble in water than the neutral gelatin 
and hence the liquefaction of these systems.” 

Fischer ?° then describes experiments showing that the addition 

of a neutral salt in increasing concentration to a previously 
liquid gelatin at first increases its viscosity to an optimum point 
(gelation) and then decreases it. Just as in the case of soaps, 
the salt becomes hydrated and, as salt-water, becomes emulsified 
in the hydrated basic (or acidic) gelatin. When salt is added 
beyond the optimum point, the salt-water becomes the external 
phase and the viscosity of the system falls. When enough salt 
is added the whole of the gelatin (as sodium gelatinate or as 
gelatin chloride and not as “neutral” gelatin) separates off in 
practically anhydrous form. 

Substantially Fischer’s views agree with those of Wo. Ostwald. 
However, Ostwald *° believes that beyond a certain critical point 
swelling passes over into solution, the spatial continuity of the 


two phases relative to each other being then destroyed. 


27 Since there are many opinions regarding the nature of ‘solution,’ accurate 
definition of the term is not easy. We are here using the term in its broadest 
sense as covering everything, in the case of colloids, from their liquefaction point 
upwards to tbe accepted ‘‘true’’ solution of the physical. chemists. 

28 Loc. cit., p. 221. 

*9 ‘Handbook of Colloid Chemistry,’ 2d ed., M. H. Fischer’s translation, p. 261. 


. 


THE CHEMISTRY OF GELATIN AND GLUE 45 


Based upon ultramicroscopic evidence the writer agrees with 
Ostwald’s view. When the Brownian motion of particles be- 
comes sufficiently violent to carry them beyond each other’s 
range of molecular attraction, then the dispersion due to swell- 
ing begins to pass over into solution. With the proteins there 
seems to be no sharp line between swelling and solution, for slight 
thermal changes or mechanical action may produce sufficient 
dispersion of part of the swollen protein to produce a colloidal 
solution, which may later aggregate once more to a gel with 
larger motionless particles. Indeed with some, if not all salts, 
colloidal dispersion precedes true solution. Thus Alexander and 
Bullowa *° observed that sodium citrate, on going into solution, 
gave off streams of actively moving ultramicrons. 

Solution, then, results when the intramolecular adsorption of 
the solvent is powerful enough to force molecules or molecular 
groups beyond a certain critical distance from each other. (The 
greater the degree of dispersion of the particles, the more rapid 
the Brownian motion and the nearer the approach to true or 
molecular solution; and this is conditioned by temperature, pres- 
sure, protective substances, coagulators, etc. As dispersed parti- 
cles aggregate, the Brownian motion decreases sharply, groups 
about 1.1 u being nearly motionless. The closer particles are the 
less water they tend to adsorb. Therefore dilute jellies, when 
dried, take up more water than do concentrated jellies. Heating 
to 40° annihilates these differences for the time being, as the 
complexes are then broken down. The molecular groups con- 
stituting gelatin are so large and possess such a powerful idio- 
attraction, that it is not easy to separate them. Furthermore 
the degree of separation, that is, the size of the gelatin “mole- 
cule,” seems to depend upon circumstances, as may be seen by 
considering work on the molecular weight of gelatin. 


Molecular Weight of Gelatin. 


Wide differences of opinion exist regarding the molecular 
weight of gelatin. Various methods have yielded the following: 


Schiitzenberger and Bourgeois........... — 1,836 
A cas cee he re — 900 
PrmtreneanG TUE... wk eck eee — 839 


%o J, Alexander and J. G. M, Bullowa, Arch. of Pediatrics 27, 18 (1910). 


46 GLUE AND GELATIN 


Proctor and \Wilsons. 2. <5 ie. ss yawn — 768 

Berrarigeesih cess come eee eae — 823 eee 

Biltz, Bugge and Mehler................ a rs See 
1) J OTGANs LOVE aontes oes eee maser as — 10,300 

Dak is, ov GAR OR EGR Sa ewes — 11,800 

Jeo re a ae eee — 12,000 to 25,000 

GoR. Sunith x, 58 ck roa ee ae about — 96,000 


D. Jordan Lloyd *! estimates that the molecular weight of gela- 
tin is about 10,000 or some multiple of this figure. Her evidence, 
based on chemical grounds, is given below: 
Van Slyke’s*? analysis shows the following distribution of 
nitrogen in gelatin: 
Per cent. of 


total mtrogen 
Ammonia nNItTOZeN. .ciukeess diode sie o's soe ae 2.25 
Melanine Me « Veen. gus Son tis ar giatereieosie, 6 cal ae 0.07 
Cystine 00 ABTS MS ae See aw ee ee 0.00 
Arginine PE: SME TRO i) TAPIA os ee 14.7 
Histidine. 8 oe pip wien soa 6 0 wate Go che Otenne ee 448 
Lysine (..e00 7. Sie ew eee oe ete es eee nen th tee 6.32 
Mono-amino nitrogen -.0..2).0%4.55 4. «ssdies a 56.3 
Non-amino - = proline + oxyproline.......... 14.9 


Assuming 1 histidine grouping in the gelatin molecules there must 
be 3 histidine nitrogens. The percentage histidine value, 4.48, 
can be reduced to 3 by multiplying by the arbitrary factor 
0.665; but this would yield figures corresponding to fractional 
(half) molecules for both ammonia and arginine. Assuming 2 
histidine groupings, the factor becomes 1.33 and we have 


Ammonia nitrogen *:.<. 5.44.00 2.9 approximately 1 x 
Arginine bye Me Top Teak CSA 19.7 4X5 
Histidine AER PE Ron: Rye 6.0 t be A 
Lysine CTE he ah el eg eee 8.4 s 2X4 
Mono-sdming (ici Go.e eect 152 e 1 X 76 
Proline + oxyproline nitrogen..... 20.0 oi 1 X 20 


These figures indicate that 1 gelatin molecule contains 133x 
nitrogen atoms distributed thus: 


3x Ammonia (amide) groupings 4x Lysine groupings 
2x Histidine 76x Mono-amino : 
5x Arginine i 20x Proline + oxyproline “ 


In the absence of other evidence, x may be taken as unity. But 
the total nitrogen in dry gelatin is 18.0 per cent. of the total dry 
weight, as is evident from the following analysis: 


51 Biochem. J. 14, 166 (1920). 
32 J, Biol. Chem. 10, 15 (1912). 


THE CHEMISTRY OF GELATIN AND GLUE 47 


Per cent. N 
CON) es ee 18.3 —Annalen, 45, 63 (1848). 
Chittenden and Solley.......... 18.0 —J. Physiol. 12, 33 (1891). 
Sad 18.12— Berichte, 25, 1202 (1892). 
SUMRMM DCMS. les sk eee es 17.81—J. Exp. Med. 2, 117 (1897). 
Schiitzenberger and Bourgeois... 18.3 —Jahresbericht Thier-Chem. 1876, 

30. 
Sadikoff (Kjeldahl method)..... 1747—Z. physiol. Chem. 37, 397 (1903). 
‘c (Dumas’ (79 ) dak 2P 18.18—“ 6c 66 (73 73 “ce 
18.0 + 2% 


“If 133 atoms of nitrogen form 18.0 per cent. of the weight 
of the gelatin molecule, then the lowest weight of gelatin which 
can act as a chemical individual must be HBP SL eaauy = x ue 10,344, 
or approximately 10,300. The error in the mean of the analyses 
given above falls within 2 per cent.; the error in van Slyke’s 
analyses is of the order of 1 per cent.; the total error in the com- 
puted value is therefore of the order of 3 per cent.” 

C. R. Smith ** working with highly purified ash-free gelatin, 
found at 35° an osmotic pressure of approximately 48 mm. of 
water for a concentration of 2 grams (1.78 dry) gelatin per 
100 cc. water, and 95 mm. for 4 grams (3.56 dry) per 100 cc. 
On the assumption that the gas laws apply, this indicates for 
gelatin molecular weight of about 96,000. 


The Crystallization of Gelatin. 


P. P. von Weimarn ** claims to have crystallized both gelatin 
and agar. He maintained a very dilute solution of gelatin in 
aqueous alcohol at 60°—70° in a dessicator containing dry potas- 
sium carbonate which absorbs water vapor but not alcohol 
vapor. As the concentration of alcohol slowly increases, the 
gelatin separates out in “crystals.” C. R. Smith ** reports that 
this method failed in his hands. 

It is to be noted that this method, similar to that by which 
Hofmeister crystallized egg albumen, involves an extremely slow 
aggregation of the constituent particles of the gelatin, during 
which their aggregation tendencies may have opportunity to 
establish themselves. In this respect it is analogous to the 

83 J, Am. Chem. Soc. 43, 1350 (1921). 


34P. P. von Weimarn, “Grundziige der Dispersoid Chemie,’ 1911, p. 106. 
34a J, Am. Leather Chemists’ Assoc., Oct., 1922. 


48 GLUE AND GELATIN 


deposit of quartz crystals from silicious waters, and is free from 
the criticism that must attach to “crystallization” of colloids in 
the presence of electrolytes. For just as colloids exercise a 
powerful influence on crystallization ®® so too do crystalloids 
tend to give colloidal gels a definite form or orientation.*® Thus 
the results of S. C. Bradford,’ who claims to have crystallized 
gelatin in the presence of mercury salts, are open to doubt. So 
also are von Weimarn’s results, for he did not use ash-free 
gelatin. 

Even when gelatin slowly dries, between a slide and a cover 
glass for example, there seems to be registered an attempt 
towards orientation; dendritic forms -appear which have been 
described by Liesegang.*® 

Contrary to what is commonly believed, colloids do diffuse, 
albeit but slowly. Too little detailed reference is made to the 
classic work of Graham,*® who clearly brought out this feature. 
Thus he says *° that tannic acid passes through parchment paper 
about 200 times slower than sodium chloride; gum arabic 400 
times slower. “The separation of colloids from ecrystalloids by 
dialysis is, in consequence, generally more complete than might 
be expected from the relative diffusibility of the two classes of 
substances.” At the outset of his paper, Graham says: “The 
range also in the degree of diffusive mobility exhibited by dif- 
ferent substances appears to be as wide as the scale of vapor 
tensions. ‘Thus hydrate of potash may be said to possess double 
the velocity of sulphate of potash, and sulphate of potash again 
double the velocity of sugar, alcohol, and sulphate of magnesia. 
But the substances named, belong all, as regards diffusion, to 
the more ‘volatile’ class. The comparatively ‘fixed’ class, as 
- regards diffusion, is represented by a different order of chemical 
substances, marked out by the absence of the power to crystal- 
lize, which are slow in the extreme. Among the latter are hy- 
drated silicic acid, hydrated alumina, and other metallic per- 
oxides of the aluminous class, when they exist in the soluble 
form; with starch, dextrine, and the gums, caramel, tannin, gela- 


85 J, Alexander, Kolloid Z. 4, 86 (1909). 

%°R, EK. Liesegang, Kolloid Z. 7, 96 (1910). 

87 Biochem J. 14, 91 (1920). 

38 R. H. Liesegang, Kolloid Z. 7, 306 (1910). 

39 Phil, Trans. Roy. Soc. London 151, 183-224 (1861). 
40 Tbid., pp. 213-217. 


THE CHEMISTRY OF GELATIN AND GLUE 49 


tine, vegetable, and animal extractive matters. Low diffusibility 
is not the only property which the bodies last enumerated possess 
incommon. They are distinguished by the gelatinous character 
of their hydrates. Although often largely soluble in water, they 
are held in solution by a most feeble force. They appear singu- 
larly inert in the capacity of acids and bases, and in all ordinary 
chemical relations. But, on the other hand, their peculiar physi- 
cal aggregation with the chemical indifference referred to, ap- 
pears to be required in substances that can intervene in the 
organic processes of life. The plastic elements of the body are 
found in this class. As gelatine appears to be its type, it is 
proposed to designate substances of the class as colloids, and to 
speak of their peculiar form of aggregation as the colloidal con- 
dition of matter. Opposed to the colloidal is the crystalline 
condition. Substances affecting the latter form will be classed 
as crystalloids. The distinction is no doubt one of intimate 
molecular constitution.” 

We should not be surprised that colloids can be crystallized, 
for we now know that all substances may exist in either the col- 
loidal or the crystalloidal state, depending upon conditions. 
Where molecular mobility is great and capable of self-expression, 
visible crystals are formed, whereas where crystallization is in- 
hibited, as with glass, soaps, chilled metals, etc., the colloidal 
state tends to appear and persist. E. Hatschek ** has described 
the peculiar properties of camphorylphenylthiosemicarbazide 
whose suddenly chilled 5 per cent. alcoholic solutions form col- 
loidal gels which gradually become crystalline. W. B. Hardy *# 
had similar results with azomethin. 

The effect of temperature on the aggregation of gelatin par- 
ticles is shown by C. R. Smith,** whose work indicates a 
difference between gelatin dried at above 35° and that dried at 
below 15°. 

Some idea as to the relative size of the groups in the case of 
1 per cent. gelatin solution may be gained from the following 
table taken from H. Bechhold,** which shows in decreasing order, 
the sizes indicated by ultrafiltration experiments: 


41 Kolloid Z. 11, 158 (1912). 

4 Proc. Roy. Soc. 87, 29 (1912). 

4.7. Am. Chem. Soc. 41, 185 (1919). 
44“‘Colloids in Biology and Medicine,” p. 99. 


50 GLUE AND GELATIN 


SUSPENSIONS 

Prussian Blue 

Platinum-sol (Bredig) 

Ferric oxide hydrosol 

Casein, in milk 

Arsenic sulphide hydrosol 

Gold solution, Zsigmondy’s 

No. 4, about 40 uu 

Bismon (Colloidal 
oxide), Paal 

Collargol (colloidal silver), 
von Heyden, 20 uu 

Gold solution, Zsigmondy’s 

No. 0, about 1-4 uu 

1 per cent. gelatin solution. 


bismuth 


1 per cent. hemoglobin solution 
mol. wt. about 16,000. 
Serum albumin. 
Diphtheria toxin. 
Protalbumoses. 
Colloidal silicie acid. 
Lysalbinic acid. 
Deutero-albumose A. 
Deutero-albumose B, mol. wt. 
about 2,400. 
Deutero-albumose C. - 
Litmus. : 
Dextrin, mol. wt. about 965. 
CRYSTALLOIDS. 


Gelatin lies in the heart of the colloidal zone, and it is interest- 
ing to compare its superior water-taking capacity with that of 
dextrin which has much smaller particles. 


Chapter 4. 


The Chemistry, Physical Chemistry and Colloidal 
Chemistry of Gelatin and Glue (Continued). 


Is Gelatin a Distinct Chemical Entity? 


The great variation in the analyses of gelatin, and the diversity 
in the results of experimenters, naturally raises the question as 
to whether gelatin is a distinct chemical entity. Most experi- 
ments have been made on gelatins which have been very loosely 
described, if indeed any description is given at all. Thus D. 
Jordan Lloyd used ‘‘Coignet’s Gold Label Gelatin” and Jacques 
Loeb used “Cooper’s Gelatin.” These descriptions, even though 
fortified by determinations of ash, and hydrogen ion concentra- 
tion, give no idea as to the chemical nature of the gelatin experi- 
mented with. 

Gelatin always contains considerable gelatoses and even some 
gelatones which are products of its own hydrolysis, but prac- 
tically no one reports what per cent. these are or even gives the 
jelly strength, or optical rotation (mutarotation) from which a 
rough idea as to their percentage might be figured out. It is 
safe to assert that no one has ever prepared and experimented 
with “chemically pure gelatin,” assuming that such a thing could 
be made. From the evidence at present available it seems that 
gelatin is not a definite chemical entity. 

W. M. Bayliss,t while making experiments on the action of 
trypsin on “Coignet’s Gold Label Gelatin,” made some pertinent 
observations. To a solution of gelatin, trypsin was added. Part 
was heated to 100° at once to prevent digestion and the other 
part was digested at 39° for some days. An equal amount of 


79 KCI was added to both parts, and the change in conductivity 


noted— 


US ES EGS EY 8 1 6,760 gemmhos - 
Perret GIMPOlAUT fh oc. es less kee cca dese 6 “ 
(A gemmho is a reciprocal megohm.) 


? 


1Arch, des Scien. Biologiques, Vol. XI, Supplt., p. 261, St. Petersburg, 1904. 
51 


52 GLUE AND GELATIN 


Commenting on this Bayliss says: “Gelatin behaves differ- 
ently (from caseinogen) ; it has been mentioned already that the 
presence of gelatin in a solution does not, to any degree worth 
consideration, effect its electrical conductivity. The products 
of its digestion, on the contrary, diminish the conductivity of a 
solution containing electrolytes. In the undigested mixture, in 
fact, the conductivity was practically the sum of that of the 
gelatin mixture and that of the KCl; in the digested, on the con- 
trary, it was far less, owing to the influence of the non-electrolyte 
now showing itself in the usual way. So that, therefore, changes 
of some kind take place in gelatin during digestion by trypsin 
which tend to diminish, instead of increasing, any conductivity 
due to electrolytes. 

Amino-acids are conductors to a certain degree,” but their prop- 
erties as such will not suffice to account for the great increase in 
conductivity observed. ... It appears, however, that in the case 
of gelatin, amino-acids are not produced in any appreciable 
quantity, at all events not within the first two hours of the 
action of trypsin. We must look elsewhere then for the causes. 
One of these, viz., the effect of change of physical state, has 
already been mentioned. The point next suggesting itself is that 
concerning the inorganic constituents of these proteid and related 
bodies. The balance of evidence seems to be decidedly in favor 
of the view that these constituents are, if not actually in chemi- 
cal combination with the proteid molecule, in such a close state . 
of association as to be incapable of ionization. It has not 
been found possible hitherto to prepare an unaltered proteid 
_ free from ash.* No doubt a considerable amount is usually 
present as impurity which can be separated by prolonged 
dialysis.” 

It seems that besides being a body of variable constitution, 
gelatin carries impurities which are likewise variable in kind 
and amount, and which may exercise a potent influence on experi- 
ments made with it. Most experimenters are not sufficiently 
careful in defining the moisture content of the gelatin they use, 
so that doubt exists as to the true strength of the solutions they 

2 Kohlrausch and Holborn, “Leitvermégen der Electrolyte,’ 1898. 

3C, R. Smith has prepared ash-free gelatin, which is quite a different thing 
from isoelectric gelatin, for the latter may contain neutral ash-producing salts. 


Dhéré and Gorgolewski (Compt. rend. 150, 484, 1910), made a demineralized 
gelatin which was almost ash free. J, A. 


THE CHEMISTRY OF GELATIN AND GLUE 53 


worked with. Thus J. Loeb* made no mention of the ash or 
moisture content of his gelatin; his calculations were based on 
1 per cent. solutions while in reality he was probably using solu- 
tions of about 0.8 per cent.—an error of about 20 per cent. from 
this cause alone. Even in his recent papers and book, while he 
reports ash and moisture, he does not report the quality or 
strength of the gelatin he used. 

C. R. Smith * states that ‘working with an indefinite product 
of varying jelly strength and ash content, it is not surprising to 
find few reliable measurements of its physical and chemical 
properties.” Smith, however, states that recent work points to 
the combination of acids and bases with gelatin, although he 
does not exclude the possibility of adsorption, and concludes 
that “we are justified either by reason of correctness or con- 
venience in referring to gelatin chloride, sodium gelatinate, etc.” 

The mere fact that satisfactory “stoichiometric” compounds 
have been produced with gelatins containing all sorts of ash, 
degradation and other impurities, would seem to indicate that | 
approximately uniform free or active surfaces or electrostatic 
fields lie at the basis of the experimental phenomena. 

Such considerations as those strike at the very root of the 
experiments and especially of the conclusions of those who like 
H. R. Procter, J. A. Wilson and J. Loeb ® insist on the formation 
of definite salts of gelatin. If gelatin is not a definite chemical 
entity, it is not justifiable to speak of ‘gelatin chloride” and 
“sodium gelatinate.” 

By referring to Chapter 2, it will be seen how various runs 
from different kinds of glue stock may appear in the market as 
“gelatin.” The analytical results of R. H. Bogue® show that 
while gelatins and glues have roughly the same general degrada- 
tion products, considerable differences exist between those derived 
from hide stock and those derived from bone stock, and there are 
wide variations in gelatins derived from the same class of stock. 
The analyses are given herewith, together with analyses of sev- 
eral glue proteins which Bogue purified by fourfold precipitation 
with 95 per cent. alcohol. | 

See e.g. his address before the Harvey Society, Science, N. S. 52, 451 
(1920) ; J. Gen. Physiol. 3, 89 (1920). 

4a J. Am. Leather Chemists’ Assoc., Oct., 1922. 


5 See ‘“‘Proteins and the Theory of Colloidal Behavior.” 
. ©Chem. Met. Eng. 23, 61 (1920). 


54 GLUE AND GELATIN 


It should be pointed out that even these analyses do not show 
the full extent of the variations to be expected in gelatin, for we 
must also take into account differences in the degree of hydrol- 
ysis, which begins the moment the gelatin forms, and continues 
during its manufacture and may even continue while experiments 
are being made with it. (See Chapter VIII.) 


. Hipp Give ANALYSES (BoaueE) 
Figures show per cent. of total nitrogen in each fraction 


Aver- 
A, A, Hy; Hyg Hs HH, age 

AMIMONIS PINGS wae sires 5 163 1.89 3.20 2.15 244 249 2.90 
Melanin iN c6 oe eee sett 0.53 0.50 0.74 0.53 060 0.63 0.59 
Cyetiner nos .cce tees 0.00 0.00 0.00 0.00 0.00 Trace 0.00 
Arginine&N oni 2eG a2. 13.27 1628- 13.76 13.72 13.50.1237 
Pistidimes Nie ero zat 1.3) a0 3.19 3.0L 245 1.59 2.19 
LVSINO SUN cha oe een 8.17 8.50 8.58 7.40 800° 8722 7.97 


Amino N in filtrate... 58.87 55.17 55.00 57.90 58.02 56.10 56.84 
Non-amino N in filtrate 17.00 15.53 1558 1526 15.24 1520 15638 











Total regained ..... 100.78 99.17 100.05 10027 100.25 96.10 100.02 
BonE GuuE ANALYSES 

Aver- 

By Bz Bs B, Be B, age 

Ammonia N-....... 4.43 4.49 4.57 4.49 448 5.04 4.55 

Melanin’ Nevt.+.e2 ea 0.74 1.18 1.03 0.82 0.76 0.95 0.91 

Cystane ANS) a2 are 0.00 0.00 0.00 0.00 0.00 Trace 0.00 

Arginine Nurs. sce 13.32 1282 1828 12.74 1356 Seige 

Histidine N ........ 1.60 0.54 1.52 1.44 1.58 4.02 1.78 

Tefsine oN, Bos oe as 7.18 8.23 7.18 8.57 9.42 9.13 8.28 


Amino N in filtrate. 56.90 58.15 57.30 57.58 5430 5340 56.27 
Non-amino N in fil- 
trate. eee 1621 1518 15.32 1436 1590 eit 

















Total regained .... 100.38 100.59 100.20 99.80 100.00 100.40 100.21 


PuririeD Protein, Fish GLUE AND IsiIneLAss ANALYSES 
H; Protein By Protein Fish Glue Isinglass 











AINMONIS aNgev: ees oe eer 1.33 Bey? 5.15 3.98 
Melanin WN.  eseereee se ae 0.78 0.74 Ee. 0.68 
Crstinie aN 05 sca eee 0.00 Trace - Trace 0.00 
Arginine ONO a in asa eee 12.61 10.96 13.80 14.20 
Histidine sNoc seek eee ee 0.82 2.24 2.04 2.00 
Lysine aNt got eee ae 8.34 8.60 8.58 6.06 
Amino N in filtrate....... 60.00 58.05 60.20 58.65 
Non-amino N in filtrate... 15.49 15.47 9.66 13.59 

Total regained ......... 99.37 99.63 100.55 99.49 


Bogue summarizes his conclusions as follows: 
“Hide and bone glues vary slightly in their chemical con- 
stitution on passing from grade to grade. This is interpreted to 


THE CHEMISTRY OF GELATIN AND GLUE 55 


signify that as the boiling of a glue progresses some ‘foreign 
substances’ as chondridin, keratin, mucin, etc., become hydro- 
lyzed and enter the solution. These have no value in glue, and 
by adulteration lower the value of the product. 

“Hide and bone glues differ from each other in their chemical 
constitution. This is taken to signify that the protein complexes 
from which the glues are derived are different in the two cases, 
or that the ratio of the several constituents is different. 

“Glues of different stock within both hide and bone series 
show a difference in constitution, which is attributed to varia- 
tions in the protein complexes of the several stocks. 

“The differences between hide and bone glues are found in the 
protein fraction to a lesser extent, and in the proteose-peptone 
fraction to a greater extent than obtained in the whole glues. 

“Tf the purified protein from the highest grade animal glues 
may be considered as pure gelatin, then. it follows that isinglass is 
not a pure gelatin, or if the assumption be made that isinglass 
consists only of gelatin, then the purified animal glue protein 
contains impurities. 

“The lower the grade of a glue, the further is it removed in 
constitution from that of the purified protein, and, if this protein 
be assumed to consist only of gelatin, then the gelatin content 
of glues diminishes with the grade, and substance from which 
the hydrolytic products are obtained consists of gelatin in de- 
creasing amounts, as the grade decreases. 

“Fish glue corresponds more closely in its composition to low- 
grade bone glue than to any other. 

“Fish glue and isinglass show a fundamental difference from 
animal glues in their low ‘non-amino nitrogen of the filtrate’ 
(proline, oxyproline, and tryptophane).” 

S. E. Sheppard and 8. 8. Sweet * have shown that impurities 
exert a powerful influence on gelatin. They brought ash-free 
gelatin to different De values, and found that it showed maximum 


rigidity (jelly strength) at p,, 8, at all concentrations. The 
curves show a “shoulder near the isoelectric point (which they 
say is p,, 4.8) but no definite maximum, or minimum. The 


curves were greatly altered by traces of aluminum salts, enough 
to give as little as 0.01 per cent. of Al,O, on the dry gelatin 


7 Science 46, 28 (1922). 


56 GLUE AND GELATIN 


displacing the maximum on the alkaline side and producing a 
secondary maximum at p,, 5. 

S. E. Sheppard has just published ™ an interesting discussion 
of gelatin in photographic processes, and is actively investigating 
many collateral questions. He reports that experiments with 
F. A. Elliot show that with high and low p,, values gelatin 
shows perceptible hydrolysis even at 50° C. He also observes 
that “on washing out strong electrolytes from gelatin, it will 
tend to approach the isoelectric point, but if hydrolyzable sub- 
- stances are present, the gelatin will retain unequal amounts 
of the basic or acidic constituents, the excess depending upon 
conditions.” 


The Significance of Hydrogen Ion Concentration. 


The importance of H ion concentration (p,, value) is so 
stressed to-day, that a brief consideration of the principles in- 
volved in its determination will not be amiss. 

Pure water is slightly dissociated according to the eee 

=H, 2H’ + OH 
This is a reversible reaction, and therefore at any temperature 
an equilibrium is reached where the rate at which water mole- 
cules split into ions, equals the rate at which these ions recom- 
bine to form water. The Law of Mass Action® demands that 
the rate of ion formation depends upon the concentration of un- 
dissociated water (C H,0)? while the rate of ion recombination 


is proportional to the product of the concentrations of positive 
and negative ions (C ,,. XK Co,,-). 

Therefore Cio as Oe 4 OF 

Cy X Con 


or ——_._—- = a constant value callea' kh: 
Ci O 
2 
But with water under ordinary conditions the dissociation is so 

small that the amount of undissociated water may be Bene 
to remain constant. That is 

2 X Coy =k X (Cyo) = K 
the constant of snes for water. — 


7a J, Ind. & Eng. Chem. 14, 1025 (1922). 
§’ How far the Law of Mass Action may be applied to colloidal solutions is 


still an open question, 


THE CHEMISTRY OF GELATIN AND GLUE 57 


By several methods the value of Khas been to be 10% at 
2°; that is C,, = C,,, = 10%, which means that in pure water 


there is a concentration of 1/10,000,000 for both H and OH ions. 
Assuming that the Law of Mass Action continues to hold, if 


C,, 1s increased by addition of an acid or acid salt which disso- 


ciates in water, the value of C AE must diminish proportionately, 
since C,, XC, yy Must remain constant. Correspondingly, the 


ace of an Seat or an alkaline salt diminishes the number 
of H ions.® 
Therefore a solution is acid when C,,>10%, or C,,, <10", 


au alkaline when O10? or Cte >107. It must be remem- 


bered, however, that the deviation of H or OH ion concentra- 
tion from the value 107 depends upon the extent to which the 
added acid or basic substance is dissociated, and represents 
therefore not the total acidity or alkalinity but rather the 
aes reaction, 1.e. the degree of acidity or alkalinity. Thus 


while 7 HCl and 0 acetic acid will each neutralize equivalent 


quantities of — ” NaOH, their respective H ion concentrations at 


10 
18° are given in the following table from Michaelis." 


Degree of Normality Cy+ Equivalent p jy value 
MGhloo S005 tiaras 1.0 8.0 X 107 0.10 
0.1 84 X10? 1.07 
0.01 9.5 xX 10* 2.02 
0.001 9.7 X 107 3.01 
0.0001 98 10- 4.01 
Acetic Acid ...... 1.0 43 X-10° aad 
0.1 - 1.36 X 10° 2.87 
0.01 A310 oe 
0.001 1.36 X 10% 3.87 
DTS |S ae 1.0 0.90 X 10% 14.05 
0.1 0.86 X 10°* 13.07 
0.01 0.76 X 10” 12.12 
0.001 0.74 X 10 T1138 


®“'The discrepancies observed, especially in strong acids, between the ionic 
concentrations as measured by conductivity methods on the one hand and with 
the hydrogen electrode on the other, suggest that the quantity which we call 
hydrogen ion concentration may not actually represent the degree of normality 
of hydrogen ions in the solutions under test. Some have preferred to call 
this quantity ‘activity.’’’ Leeds and Northrup Co, Catalog 75 (1921), p. 6. 
10 “‘Wasserstoffionen Konzentration.” 


58 GLUE AND GELATIN 


As the table shows, HCl is a “strong” acid and acetic a “weak” 
acid, as measured by effective reaction. But since the H* and Cl 
part company and ionize more readily than do H* and CHCOO’, 
it is evident that the latter bond is less readily relaxed in 
aqueous solution; so that acetic acid is really stronger from 
this point of view. This means that the anion largely if not 
entirely controls the p,, value of an acid. Therefore even if as 
J. Loeb claims, the p,, value be the main factor controlling the 
swelling and general colloidal behavior of gelatin through the 
Donnan equilibrium, the wltxmate cause is to be found in the 
specific nature of the anions of acids or the cations of bases, 
because they control the H ion concentration (p,,). They thus 


form the raison d’étre of the Hofmeister series. 

Thomas and Baldwin! showed that salts change the Pass 
In the case of hydrochloric and sulphuric acids, sodium chloride . 
increases the p,,, whereas sodium sulphate decreases it. These 


measurements were made after waiting two days. 

Since it is Inconvenient to express and read H ion concentra- 
tions by products of the character given in the table above, 
Sgrensen proposed to perform the multiplication logarithmically 
and use the logarithm of the product after dropping the minus 
slgn. | . 

Thus an acidity 10 times as great as that of pure water (that 
is C,, = 10 X 10°‘) would be represented by log 10 + log 107 = 
1 — 7 = — 6; and dropping the minus sign, p yz (Sorensen’s ex- 
pression) = 6. In hke manner an acidity 100 times greater than 
that of water would be C,,= 100 XK 107’ =2—7—=—45, ‘or 


eats 
Number of times H (or OH) ion concentra- 
PHY value tion exceeds that of pure water 

Le ee ee ee 1,000,000 
Dn ha aig iatema ee 100,000 
FE meet Sec 10.000 
2 PA Gye Ee A NES He eo 1,000 Beis 
ep eiae pete do 100 acid side 
Geer ck ee ante 10 
Po i EGS SF ae ae Q pure water .....07..4 eee 
pees ON ey 10 J 
OUR eS eet eee 100 alkaline side 
LOS eee ee eee 1,000 
LTRS BOL eee 10,000 
1D 490 2 beer 100,000 


10a J, Am. Chem. Soc. 41, 1981 (1919). 


THE CHEMISTRY OF GELATIN AND GLUE 59 


This table shows two facts which must not be forgotten; first 
that an increase in p,, means a decrease in H ion concentration, 


and second that the acidity does not decrease numerically with 
the p,, value but decreases in logarithmic ratio. It is obvious, 


therefore, in plotting experimental results, that the P,; values 


should be reconverted into true values, i.e. H-ion concentration, 
or else logarithmic paper should be used. If the p,, values are 


laid off numerically as abcissas or as ordinates, the resulting 
curve will be logarithmically compressed, and the presence of 
inflections or of cusps in the true curve may thereby be over- 
looked. In any event such curves give a wrong idea of the 
relative hydrogen ion concentrations." : 
The table shows another point of great interest which is often 


overlooked, which is that p,, = 4.7 (the isoelectric point of 


gelatin), represents an extremely slight acidity. Thus ordinary 
distilled water prepared in the laboratory still, has a p,, of 


about 5.5, but when boiled to expel the CO, absorbed, the reac- 


tion drops to Py =7 (neutrality). On the other hand reel 


has ap, = only 2.02. The most striking changes with gelatin 
occur between about Py 2 and Py + on the acid side, and 
Pz —9 and p,,—11 on the alkaline side.’ These represent 


comparatively weak acidity or alkalinity; but with a slightly 
ionized acid like acetic, so much acid must be used to produce 


a Pp , = about 2, that the specific solubilizing action of the acid 


on gelatin becomes very marked.1*? 


1 Many of J. Loeb’s experimental results are given solely in erroneous curves 
of this character, and should be given in tabular form or the curves redrawn. 
For full details regarding apparatus for determination of H ion concentration, 
the formulas whereby the electrical readings are converted into Cy,, and the 


precautions to be observed, the reader is referred to standard texts, and espe- 
cially to W. M. Clark’s book, “The Determination of Hydrogen Ions,” Balti- 
more, 1920. 

122Thus C. R. Smith found that 1 gram of air dry ash free isoelectric gelatin 
swelled in pure water to 7-8 ec., while the maximum acid swelling was 48 cc. 
and the maximum alkaline swelling was 30 ce. Low jelly strength gelatins 
give decreased swelling.—Smith, J. Am. Chem. Soc. 43, 1860 (1921). 

1za See e.g. J. Loeb, ‘‘Proteins,”’ p. 80. 


60 GLUE AND GELATIN 


The Titration Curve of Gelatin. 


Dorothy Jordan Lloyd and C. Mayes,'* in order to determine 
- the amount of HCl or NaOH which would combine with a cer- 
tain weight of gelatin, determined potentiometrically the dif- 
ferences between the H ion concentrations of 1 per cent. gelatin 
solutions containing different percentages of acid and alkali. 


The gelatin was Coignet’s Gold Label, reduced to Py = 46, ash 


between 0.00 and 0.06 per cent. Knowing the H ion concentra- 
tion of equally concentrated systems containing no gelatin, they 
calculated by the formula of Blasel and Matula‘™ the concen- 
trations of HCl removed by the gelatin from independent solu- 
tion. 

The Blasel and Matula formula is based on the erroneous 
assumption that the ionization of HCl is the same whether or 
not the normality of H and Cl are identical, so they made a 
correction for this, using the Cl ion concentration as a factor. 
But the value of the Cl concentration is based on the further 
assumption, which was also made by Procter and Wilson *° that 
“gelatin chlorid” is completely dissociated. This latter assump- 
tion is not supported by.the figures of Bugarsky and Lieber- 
mann, on Cl ion concentration.t** Jordan Lloyd and Mayes 
remark that this assumption is liable to lead to an increasing 
error with higher H ion concentration. | 

On plotting their results with the normality values of H ion 
concentration as abscissas and the amount of HCl fixed as 
ordinates, they obtained not a simple smooth curve, but one con- 
sisting of two, possibly three distinct regions, indicating that 
“up to a given concentration of hydrogen ions, a group of 
hydroxyl ions having approximately equal ionization constants 
is involved; beyond this concentration, and up to a second fixed 
value, a second group approximating to a second constant is 
involved; and beyond this again there is slight evidence of a 
third group. The factors required in order to bring the second — 
and possible third groups into conformity with the generalized 
statement of the law of mass action are not yet fully known.” 


28 Proc. ROY AS 0G. By 93, .09NGLo22) 
14 Biochem. Z. 58, 417 (1914). 

1% Trans. Chem. Soc. 109, 307 (1916). 
iba Pfliiger’s Arch, 72, 51 (1898). 


THE CHEMISTRY OF GELATIN AND GLUE 61 


Their results for the amount of NaOH fixed had greater experi- 
mental errors than did the determination of acid fixation, for 
the gelatin was probably attacked by the alkali in the presence 
of the spongy platinum of the electrode. The curve was made 
by using normality values of NaOH as abscissas and the amounts 
of NaOH fixed as ordinates. The curve rises abruptly, appar- 
ently seeking a maximum when OH- = about 0.005 N, but then 
begins to rise sharply again, giving a very steep curve. “Hence 
it is obvious that in alkaline solution gelatin does not behave 

simply as a weak acid dissociating in accordance with the law 
of mass action. It is possible that this abrupt rise accompanies 
some structural change of the protein molecule such as Dakin 
had shown to occur in strong alkaline solution.” 1° 

Jordan Lloyd and Mayes then discuss the mechanism for the 
fixation of ‘acid and alkali. While in solutions of HCl less than 
0.02 N it is possible that gelatin may bind the acid by its free 
amino-groups, “with increasing concentration of acid, more acid 
is bound than can be accounted for on this hypothesis, and it is 
therefore necessary to consider what part the imino-nitrogen 
of the peptide linkage (— COHN —) could play. Robertson *" 
states that the acid binding properties of the proteins are not 
much increased by hydrolysis, and we have found that the reac- 
tion of a 1 per cent. solution of gelatin, which was found to be 
Py, = 1.8, had only changed to Py = 112 after 12 hours at 


100° C. This change is of the same order as the experimental 
error of the method, nevertheless hydrolysis of the gelatin had 
occurred during the heating in the strong acid solution, as was 
shown by the fact that the gelling power had been destroyed. 
It seems, therefore, that the peptide linkage can function as an 
acid-binding group. ...It seems clear that some of the 
— COHN — groups can act as basic groups combining with acids. 
What role, if any, other groups (such as the hydroxyl groups 
of the hydroxy acids) in the molecule play in acid fixation is 
still unknown. It will be necessary to follow experimentally the 
fate of the chlorine ion before final decisions are possible. . . . 
The theory that proteins fix bases by means of their free carboxyl 
groups has given way on accumulation of evidence that there 
are not enough of the latter to explain the quantitative reactions. 


16 J, Biol. Chem. 13, 357 (1912-13). 
wz “The Physical Chemistry of the Proteins.” 


62 GLUE AND GELATIN 


... The possibility of linkage at some of the hydroxy groups of 
the substituted amino-acids, serine and hydroxy-proline, is not 
to be ignored. Hydrolysis of gelatin by caustic soda has been 
shown to increase slightly its basic binding power, a fact which 
suggests that not all the — COHN — linkages are as potent as 
base fixers as the free —COOH — groups. Loeb?® has shown 
that bases react with gelatin at the same hydroxyl ion concen- 
tration in equivalent proportions. This fact shows that the 
reaction is lonic, and that the compounds formed are of the 
nature of ionizable salts. Loeb only worked with solutions 
whose alkalinity is less than p,, = 9. His experimental values 


correspond very closely to our values over the same range. . 
There is both qualitative and quantitative evidence to show that 
in the same protein (gelatin) the mechanism of fixing acids is 
different from that of fixing bases.” 

D. I. Hitchcock *** determined the combination of gelatin 
with hydrochloric acid. Using 1, 24%, and 5 per cent. gelatin 
solutions with varying acid content, he subtracted the ise 


values of the acid-gelatin solutions from those of solutions con- 
taining equal amounts of pure acid, the difference being con- 
sidered as indicating the amount of acid combined with the 
gelatin. He reports that about 0.00092 mol. of hydrochloric 
acid combine with 1 gram of gelatin between Pj, 1 and 2, but 
found no evidence of a discontinuous section in the titration 
curve, as did Lloyd and Mayes (vide supra). 

Oakes and Davis?® found a definite relationship between the 
grade of a gelatin and the amount of acid required to titrate it 
over the range Pj, 4.7 to p,, 3.5. To make this change in reac- 
tion they report as follows: 


Jelly Strength “Molecular Weight” 
Gelatin Py 423°C. cc.02M HCl of Gelatin 
er iste tee cee eres 59 Broce 1,319 
Din Ne 5 get 430 2.85 1.753 
Digg ee ake is whee 565 2.70 1,852 
ae eels ose ee Ripa 800 2.48 2,016 
MI ay Sean te Mex ag Rote op 1,025 2.40 2,083 


J. Gen. Physiol. 1, 379, 487 (1919). 
a J, Gen. Physiol. 4, 7383-9 (1922). 
19 J, Ind. Eng. Chem. 14, 706 (1922). 


THE CHEMISTRY OF GELATIN AND GLUE _ 68 


They think these figures ‘indicate the order of magnitude of 
the molecular weight, and its progressive increase with the grade 
of gelatin,” although they state elsewhere in the same paper: 
“Since there is probably no gelatin that is not made up of a 
series of the products of hydrolysis of the original tissue, no 
gelatin can have what may be called molecular weight. What 
is determined is the mean molecular weight of all the various 
fractions making up the gelatin sample.” Nevertheless they 
believe that a (presumably chemical) compound is formed at 
the maximum point of the viscosity—p ,, curve. 


Since Oakes and Davis do not give the ash content of these 
gelatins and since lower grade gelatins often contain more ash 
than higher grades, it is probable that the variations in the above 
table are to some extent due to the ash. T. B. Robertson found 
that profound hydrolysis did not materially change the acid 
fixing capacity of a gelatin. Furthermore the increase in vis- 
cosity of isoelectric gelatin by the addition of acid, is readily 
accounted for by swelling of its complexes without assuming the 
formation of a gelatin “salt.” Oakes and Davis also state: 
“The difference in ash content of gelatin is, then, the main cause 
for the lack of agreement between classifying gelatins by vis- 
cosity and jelly strength measurements, and for a given ash 
content viscosity measurements may be substituted for jelly 
strength measurements.” ‘The importance of ash depends not 
only on its amount, but also upon its composition; in any event, 
as above indicated, it cannot be left out of consideration. 

The fact that profound hydrolysis as well as deaminization do 
not materially alter the acid and base fixing power of gelatin, 
is an indication that definite gelatin salts and metal gelatinates 
do not exist, a view further strengthened by the irregularities 
of acid and base fixation. While the theory that such com- 
pounds exist may be made to fit many of the experimental facts 
by assuming conveniently variable dissociation values, it seems 
more likely to the writer that the fixation of acid and alkali 
should be regarded as due to adsorption, the extent of which 
varies as the total free or effective adsorbing surfaces of the 
molecular complexes are changed by varying conditions, one of 
which is hydrogen ion concentration. 

How do H* ions or OH ions function when they produce 


64 GLUE AND GELATIN 


initially an increased swelling of isoelectric gelatin which has 
already been swollen to its limit in isoelectric water of p,,=7? 


Even ash-free isoelectric gelatin swells considerably in pure water 
P yz — 7, proving that its “molecules” or molecular groups have 


considerable residual attraction, which enables a high-class air- 
dry gelatin to hold about 20 per cent. of water that can be driven 
off by drying at 110°. Obviously with an acid or alkali the ions 
are concentrated within the gelatin particles, for the concentra- 
tion of the external solution (unless in too great an excess) is 
diminished. 

The original view of W. B. Hardy 2° (also accepted by J. 
Perrin) was that “as the H and OH ions have by far the highest 
specific velocity the colloidal particle will entangle an excess of 
H ions in acid and thereby acquire a + charge and of OH ions 
in alkali and thereby acquire a — charge. These charges will 
decrease the surface energy of the particles and thereby lead to 
changes in their average size.24_ This would mean that, in the 
kinetic equilibrium existing, H (or OH) ions would accumulate 
within the “molecule” of gelatin, and cause its distension by 
electric repulsion along the lines suggested by Tolman and 
Stearns. 

Hardy ”? later regarded the H and OH ions as being held by 
chemical attraction, but pointed out that “though one may speak 
of the colloid particles as being ionic in nature they are sharply 
distinct from true ions in the fact that they are not of the same 
order of magnitude as are the molecules of the solvent, the 
electric charge which they carry is not a definite multiple of a 
fixed quantity and one cannot ascribe to them a valency, and 
their electrical relations are those which underlie the phenomena 
of electrical endosmose.”’ 

Even though Hardy ** express the view that proteins form 
salts with acid and alkalis, he expressly points out that “the 
reactions are not precise, an indefinite number of salts of the 
form (B)n BHA being formed where the value of n is deter- 


2 J. Physiol. 29, 29 (1903). 

21 Very likely the small size as well as the speed of these ions is also a factor 
and as H- is smaller and speedier than OH- we should not be surprised if we 
find that the minimum swelling of gelatin is not in pure water, but very 
slightly on the alkaline side of the isoelectric point. J. A. 

22, J, Physiol. 22, 251 (1905) ; Proc. Roy. Soc. 79, 413 (1907). 

2'T, B. Wood and W. B. Hardy, Proc. Roy. Soc. 81, 38 (1909). 


THE CHEMISTRY OF GELATIN AND GLUE _ 65 


mined by conditions of temperature and concentration, and of 
inertia due to electrification of internal surfaces within the solu- 
tion.?* 

It seems, as Tolman and Stearns suggest, that the H (or OH) 
ions, adsorbed at the free surfaces of the micellular groups, dis- 
turb whatever balance exists at the isoelectric point, and the 
resulting -- (or —) charges at these free surfaces cause repul- 
sions which still further distend the gelatin. 

But how shall we account for the fact that after reaching a 
maximum at about p,, = 3, further additions of acid or alkali 


cause contraction again? In higher concentrations the “salting 
out” action of the acid or alkali dehydrates the gelatin and 
causes shrinking. A. Kuhn,?> who investigated the swelling of 
gelatin in over fifty aliphatic and aromatic acids, found that the 
swelling is controlled by four factors: 


A 1. Individual Swelling or Hydration. 


eure . | increasing 
B He a ae (Sol formation) i ociinenieadiin 
nein | of acid. 


C 4. Dehydration, or Flocculation. 


The maximum is determined by groups A and B, and is defined 
as the point where the increasing swelling or hydration due to 
rising acid concentration is overbalanced by sol formation and 
hydrolysis, which also increase at the same time.?° 

Since the forces which govern adsorption are molecular, i.e. 
due to residual atomic fields of force (see Chapter II, p. 28), it 
is natural that they should be greatly influenced by the chemical 
constitution of the molecular groups which form the gelatin 
“molecule.” Part of these residual fields are balanced in hold- 
ing the molecular groups together as the gelatin “molecule,” but 
the remaining moieties are free to attract and hold ions or groups 

*4 According to R. Keller (Kolloid Z, 27, 255 [1920]), the degree of dispersion 
exerts a vital influence on the chemical and electrical properties of colloids. 
Thus very dilute solutions of methylene blue move to the cathode despite addi- 
tion of alkali; but in somewhat coarser dispersion it moves partially or entirely 
to the anode. The reversal of charge of colloids is not to be expressed in terms 
of H ion concentration, for chemical combination of stoichiometric character 
hardly exists between colloids and ions whose weight and volume are roughly 
as 1,000,000,000 to 1. J. A. 

25 Kolloidchem. Beihefte 14, 147 (1921). 


*6 Kuhn’s experiments must be repeated for they were made with gelatin con- 
taining 3.13 per cent. of ash. 


66 GLUE AND GELATIN 


having an opposite charge, and thus “fix” acid, alkali, etc., in 
proportion to the effective free fields of the acid and alkali, 
which of course vary stoichiometrically; that is, according to 
their respective effective valencies. Therefore the fixation of 
acids and bases in their stoichiometrical ratios is no proof that 
a definite chemical compound has been formed, i.e. that the 
gelatin has combined stoichiometrically. Nor is it surprising 
that on the acid side of the isoelectric point, when gelatin has a 
net positive charge it should act as an acid and vice-versa. 

As Wo. Ostwald 2? puts it, in adsorption there comes into play 
not the stoichiometric mass, but the active mass, which means 
the sum of the chemically active surface layers. When true 
molecular or “crystalloid” dispersion exists, the ratio between 
these two is unity; but in colloidal aggregation the active mass 
is only a fraction of the stoichiometric mass, the value of the 
fraction depending on the size of the particles of the colloid, i.e. 
on its specific surface. Furthermore according to N. Schilow,”* 
adsorption of electrolytes depends not only on the sign of the 
adsorbent, but on the nature of the electrolyte and solvent, that — 
is on the collective properties of the system. He was able to 
reverse some adsorption series merely by small additions to 
the solvent. 

Furthermore the Donnan equilibrium and its consequences 
which J. Loeb 2° relies upon to prove the formation of definite 
chemical compounds, are just as well explainable on the basis 
of a kinetically balanced adsorption, as on the basis of “chemi- 
cal compounds” which hydrolyze. Loeb says (loc. cit., p. 63): 
“It can be stated as a result of all these titration experiments, 
that the ratios in which acids and bases combine with proteins 
are identical with the ratios in which acids and bases combine 
with crystalloids. Or, in other words, the forces by which gela- 
tin, egg albumen, and casein (and probably proteins in general) 
combine with acids and alkalis are the purely chemical forces of 
primary valency.” 

Now while the first statement is Justified by the fact that 
simple ions like Cl-, and Na* always combine according to their 
_valence fields of force, the second statement is unwarranted, and 
** Kolloid Z. 80, 254 (1922). 


22 Z. physik. Chem. 100, 425 (1922). 
22 “Proteins.” 


_-THE CHEMISTRY OF GELATIN AND GLUE 67 


is not putting the first statement in other words; for the second 
statement assumes that the gelatin also combines with primary 
valence forces, which is not the case. The gelatin compounds 
lack the precise and definite character connoted by the present 
meaning of the expression ‘chemical compound.” 


Chapter 5. 


The Structure of Gelatin Solutions and of Gelatin 
Jellies. 


The structure of jellies has long been a moot question, and 
is not yet settled. In an historical review Zsigmondy * says that 
the oldest theory assumed a porous structure for distensible 
bodies; water penetrating the pores, was held by capillary or by 
molecular attraction, and thus produced swelling. 

In 1858 Nageli? advanced his micellular theory in which dis- 
tensible bodies were assumed to be made up of tiny anisotropic 
crystal-like aggregations of molecules called micells, which retain 
their identity in solution. The micells are surrounded by a 
layer of water whose thickness is limited by the fact that the 
attraction of the micells for each other finally dominates the 
attraction of the micells for water. The swelling caused by the 
penetration of the water into the micellular mass thus reaches 
an equilibrium which may be shifted by changes in temperature, 
pressure, etc. Frankenheim * had also expressed similar views. 
O. Biitschli * advanced what is known as the honey-comb theory. 
His first experiments ° included soap solutions and emulsions of 
oils. By hardening gelatin jellies with alcohol or chromic acid, 
Biitschli was able to demonstrate microscopically a honey-comb 
structure; but it is probable the structures demonstrated are in 
this case artifacts produced by the action of the hardening 
agents ® used with the intention of rendering visible a structure 


1R. Zsigmondy, ‘““The Chemistry of Colloids,” p. 68, trans. by E. B. Spear, 
1927; 

2C¢. von Nigeli and S. Schwendener, ‘‘Das Mikroscop” (2d ed.), Leipzig, 1877; 
C. von Nigeli, ‘‘Theorie der Girung,’’ Munich, 1879. 

3**Die Lehre von der Kohision,”’ Breslau, 1835. 

40. Biitschli, ‘“‘Ueber den Bau quellbarer Koérper usw.,’’ Géttingen, 1896; 
“Untersuchungen itiber Structuren,”’ Leipzig, 1898; ‘‘Untersuchungen itiber die 
Mikrostruktur kiinstlicher und natiirlicher Kieselsiiuregallerten,” Heidelberg, 
1900. 

5“ Untersuchungen tiber mikroscopische Schiume und das Protoplasma,’”’ Leip- 
zig, 1892. 

° See H. Bechhold, ‘Colloids in Biology and Medicine,” trans. by J. G. M. 
Bullowa, Ch. XXIII, New York, 1919; also W. Pauli, ‘‘Der Kolloidale Zustand 
und die Vorgiinge in der lebendigen Substanz,”’’ Brunswick, 1902. 


68 


THE STRUCTURE OF GELATIN SOLUTIONS _ 69 


already existing. Biitschli’s view was supported by G. Quincke,’ 
who stresses the effect of the surface tension existing between 
the “oleaginous” phase and a second phase richer in water. 
Quite similar is the view of W. B. Hardy,® who considered gelatin 
jellies to consist of two phases, one a solution of gelatin in 
water, the other a solution of water in gelatin. 

Van Bemmelen, though first leaning to the micellular theory 
of Nageli, later agreed with Biitschli, whose work indicated the 
following dimensions for the diameters (d) of the tiny cavities 
in silica-gels, and for the major limits of thickness of their cell 


walls (m): 
Substance d m 
US 1.45u 0.152-0.187 
(from bamboo nodes) 
van Bemmelen’s silica gel.. 1.00u 0.27 
Biitschli’s silica gel....... 1.50u 0.30u 


Zsigmondy °® and his pupil, W. Bachmann, hold what may be 
termed the fine grained theory, which does not materially differ 
- from that of Nageli*° Zsigmondy believes (loc. cit., p. 70) that 
upon the relatively gross heterogeneity of Biitschli there is super- 
imposed a much finer discontinuity. Bachmann (loc. cit., p. 99) 
found that gelatin benzolgels and alcogels show the same type 
of curves as van Bemmelen found in silicic acid gels, in the 
_.course and hysteresis cycle of their vapor pressure isotherm. | 
“The application of the theory of capillarity to solidified gelatin 
jellies. permits an approximate calculation of their vacant spaces. 
On the average they are from 30 to 100 times smaller than the 
honey-comb spaces made visible by Biitschli in such jellies by 
coagulators. Biitschli’s honey-comb structure, whose spaces of 
700 to 800 wu are enormous when compared with the truly 
amicroscopic dimensions here concerned, can play no part as 
factors depressing the vapor tension, and are therefore not re- 
sponsible for the hysteresis cycle of a gel, which is one of its 
special characteristics.”” Bachmann therefore concludes that the 
real gel structure is much finer than Biitschli’s “honey-comb.” 

N. Sutherland + advanced what may be termed the semplar 

7 Drude’s Annalen, 1902 and 1903. 

8Z, f. phys. Chem. 33, 326 (1900) ; Proc. Roy. Soc. 66, 95 (1900). 

®R. Zsigmondy, Physik. Z. 14, 1098 (1918). 

10 See W. Bachmann, Kolloid Z. 23, 85 (1918); also Zsigmondy, “The Chem- 


isty of Colloids,” p. 127 et seq.; p. 224 et seq. 
1a Proc. Roy. Soc. 79B, 130 (1907). 


70 GLUE AND GELATIN 


theory. ‘The molecules link up by their atomic electric charges, 
forming a three-dimensional pattern or semplar, which is re- 
peated many times in each particle. | 

W. Moeller 7 considers gelatinization as a kind of crystalliza- 
tion, in which thread-like crystals traverse the jelly in every 
direction and thus form a net-like lattice of thread-like crystals. 
Based largely on microscopic and ultramicroscopic evidence, 
many investigators advance a similar view. Thus Bogue *? 
considers gelatin made up of ‘‘streptococcal threads” of molecules. 
It must be remembered, however, that the ultramicrons seen in 
the ultramicroscope, are only diffraction images of particles 
smaller than a wave length of light; they can therefore never 
be microscopically resolved. The cocci-like appearance is no 
criterion of the actual shape of the particles; and as Wo. 
Ostwald ** points out, according to the Frauenhofer-Babinet prin- 
ciple, “holes” reflect the same as discs, 1.e. both show as “grains” 
in the ultramicroscope. 

Bogue *** has elaborated his views on the structure of gels, 
besides reviewing several other theories, as well as studying 
the influence of electrolytes of varying H ion concentration, and 
of the valence of the combining ion upon gel strength, viscosity, 
swelling, foam, alcohol number and turbidity. He reports that 
the greatest opacity results from the largest aggregates of least 
swollen particles, which coincides with the view advanced on 
p. 78 as to the complex nature of gelatin particles, and with 
Alexander’s zone of maximum degree of colloidality. _ 

D. Jordan Lloyd +4 adopts and extends the general view of 
Hardy, and believes that a gelatin gel consists “of two phases, 
solid and liquid, and two chemical states of gelatin, viz. gelatin 
per se and gelatin in the form of soluble salts. Such gels there- 
fore are three component systems, the components being water, 
gelatin, and an acid (or base). ... The process of gelation is 
therefore pictured as follows: gelation will only occur on the cool- 
ing of a sol which contains in solution isoelectric gelatin, and 
gelatin salts in equilibrium with free electrolytes. As the sol 

1 Kolloid Z. 28,11 (1918). | 

122R, H. Bogue, Chem. Met. Eng. 23, 61 (1920). See also J. Am. Chem. Soc. 
44, 1843 (1922). 

1% Kolloid Z. 22, 80 (1917). 


wa J, Am. Chem. Soc. 44, 1842 (1922). 
4 Biochem. J. 14, 165° (1920). 


THE STRUCTURE OF GELATIN SOLUTIONS = 71 


is cooled the insoluble isoelectric gelatin is precipitated in a 
state of suspended crystallization and forms a solid framework 
throughout the system. The more soluble gelatin salts remain 
in solution, and by their osmotic pressure. keep the framework 
extended. Gels therefore are two-phase systems, the solid phase 
consisting of isoelectric gelatin, the liquid of gelatin in the salt 
form.” Isoelectric gelatin, therefore, where the contractile 
forces of the framework are unopposed, should be unstable and 
should squeeze out the liquid phase. Jordan Lloyd?> has pro- 
duced such a contracting clot of isoelectric gelatin and finds 
that it shows numerous small spheres about 0.5 uw in diameter, 
like the spherites described by Bradford.*® 

What Jordan Lloyd calls ‘‘suspended crystallization” is an 
indication of the protective or crystal-inhibitive action of a 
- portion of the gelatin solution, for isoelectric gelatin as Loeb 1” 
has shown is inert and insoluble in cold water. The facts sup- 
port the general view of E. Jordis,1® that electrolyte impurities 
are essential to the stability of gels. In the case of gelatin, ow- 
ing to its very high protective action (gold number), surpris- 
ingly minute percentages of hydrogen- or hydroxyl-ions are able 
to produce sufficient “gelatin salt”? to stabilize the essentially 
unstable isoelectric gelatin; and, as Jordan Lloyd’ observes, the 
purest water obtainable is still an electrolyte, and is alkaline to 
gelatin, and therefore will react with it to produce “salts.” The 
highly purified gelatin produced by Field+® was evidently not 
absolutely free from stabilizing substances, since its jelly was 
stable; it gave, however, an opaque jelly which was made trans- 
parent by traces of acid or alkali, and even by carbon dioxide 
absorbed from the air. 

The formation of a gel by Jordan Lloyd’s isoelectric gelatin 
would seem to indicate that, contrary to her contention, the for- 
mation of a gelatin salt is not necessary to the formation of a 
gel, even although the salt may be necessary to stabilize the gel 
when once formed. Hence only the dispersed substance and 
water are really essential to gel formation with gelatin, a typical 


15 Biochem. J. 14, 584 (1920). 

16 Biochem. J. 14, 91 (1920). 

7 J. Loeb, J. Gen. Physiol. 1, 41 (1918). 

18 JZ, Electrochem. 8, 677 (1902). 

19 Ada M. Field, J. Am. Chem. Soc. 43, 667 (1921). 


72 GLUE AND GELATIN 


“emulsoid,”’ as is the case with ferric hydroxide and other so- 
called suspensoids. 

H. G. Bennett *° believes that a jelly contains a continuous 
network of water under a great compression due to the con- 
tractile forces of surface tension. ‘The higher the degree of dis- 
persion of the particles, and the greater the concentration, the 
greater the proportion of the water present in the gel will be in 
the compressed state, and therefore the greater the viscosity, 
finally culminating in rigidity. Swelling is caused by the electro- 
static repulsion of similarly charged adsorbed ions, which repul- 
sion, however, diminishes more rapidly as swelling proceeds than 
does the contractile force above referred to. A balance is con- 
sequently reached, which is influenced by the “lyotrope” influence 
of dissolved substances that affect the compressibility of the 
water. 

Procter *! criticizes Bennett’s theory, and points out that his 
own theory of acid swelling (see p. 93) accounts quantitatively 
for the phenomena observed. He also makes the justified objec- 
tion that surface tension itself causes compression.?* Procter 
further observes that any existing compression cannot cause a 
large increase in viscosity, and that electrostatic repulsion is out 
of the question because the colloid particles are neutralized by 
the formation of an electrical double layer. The reason charged 
particles move in electrophoresis is that the layer is continuously 
displaced in a direction opposite to that of the motion of the 
particle. 

H. R. Procter ??* in speaking of the structure of gelatin jellies 
said: “True homogeneity can only be postulated of a hypotheti- 
cal fluid, and certainly not of any atomic structure, or even of 
the atoms themselves. The dilute solution of any substance 
must have considerable spaces of solvent between the molecules,” 
whereas “large organic chains may cohere without separation 
from the solvent, where the smaller and more definitely polar 
molecules of a crystalline substance would form rigid crystals 
in which only a definite proportion of solvent would be included 


20 J, Soc. Leather Trades Chem. 2, 40 (1918). 

217, Soc. Leather Trades Chem. 2, 73 (1918). 

22The compression nevertheless does exist, although it is due not to surface 
tension, but to the very forces that produce surface tension, namely the specific 
attractions or residual fields of force of the atoms or molecules involved. 

22a Seventh Int. Cong. of Appl. Chem., 1909. 


THE STRUCTURE OF GELATIN SOLUTIONS 73 


as water of crystallization. The true issue is therefore not 
whether jellies have a discontinuous structure, but whether the 
network is so fine that the constituents are within range of each 
other’s molecular forces, or so coarse that these forces may be 
wholly or mainly neglected.” 

Procter does not here consider that both conditions may co- 
exist, and that the “solvent”? may in addition contain selected 
portions of a complex mixture. 

F. C. Thompson ?”” believes that gelatin solutions ‘consist of 
a network of solid gelatin, molecular, or at least extremely fine, 
with pure water in the interstices.” To this view a similar 
criticism applies as to that of Procter. 

An indication that the water in gelatin jellies is “available,” 
is found in Graham’s observation that diffusion occurs in jellies 
almost as freely as in pure water, and in Dumanski’s observa- 
tions 7° that the conductivity of electrolytes shows the same 
effect. Thompson, however, regards gelatin solutions stronger 
than 0.18 per cent. as. solids because they resist indefinitely a 
small shearing strain. 

Wo. Ostwald ?* believes that the process of swelling repre- 
sents the reverse of syneresis. “The coarser structure of the 
solid is, as it were, broken up; in other words, coarse aggregates 
are divided into the primary particles of which they are com- 
posed. As N. Gaidukow ** has found the ultramicroscopic parti- 
cles of a gel become smaller in the process of swelling, or at 
least lose their highly refractive character. But in the process 
of swelling there occurs another change which may, under certain 
circumstances, actually run counter to the increase in dispersion. 
The individual particles absorb the medium in which they are 
swelling; they become solvated. ‘This increases the size of the 
particles and so fluid droplets may be formed. ‘The two changes, 
in other words, the combination of increase in degree of disper- — 
sion with a change in a type of the dispersed substance from 
the side of the solid to that of the fluid, seem most characteristic 
of the process of swelling.” 


22b J, Leather Trades Chemists’ 3, 209 (1919). 

22e J, Physic. Chem. 60, 553 (1907). 

23 “Theoretical and Applied Colloid Chemistry,’’ 1917, trans. by M. H. Fischer, 
p. 100. 

24See N. Gaidukow, ‘‘Dunkelfeldbeleuchtung und Ultra-mikroskopie in der 
Biologie und in der Medizin,” Jena, 1910; Kolloid Z. 6, 260 (1910). 


74 ) GLUE AND GELATIN 


Here Ostwald tacitly assumes the existence of a secondary 
structure in the larger particles of gels, which is also the view 
held by Zsigmondy.2> Experiments made by J. Alexander *° 
with karaya gum reduced to various degrees of fineness support 
this view; for increase in viscosity and, later on, gel formation, 
accompany the hydration and swelling of the gum fragments. 
“In fact, with hydrophile or emulsoid colloids, as the dispersed 
phase becomes less viscous by swelling, the golloids as a whole 
becomes more viscous. Viscosity may also increase as ultra- 
microns condense or aggregate, as is the case in cooling gelatin.” 
(Alexander, loc. cit.) 

Ostwald believes that gelatin gel is a two-phase liquid system. 
He says (loc. cit., p. 103): “In gels produced by swelling, I do 
not know of an instance in which the dispersed elements are 
solid or crystalline in character. They are, apparently, always 
liquid.” E. Hatschek 27 has shown mathematically that this 
theory is untenable. 

It should be borne in mind that no substance is inherently 
gaseous, liquid, or solid. These three classic states of matter 
depend upon the proximity to each other of the constituent atoms 
or molecules of the substance in question. This proximity con- 
trols the degree of their attraction, their state of aggregation or 
dispersion, and the extent or practical cessation of their kinetic 
motion. It in turn is controlled mainly by temperature, pres- 
sure, and atomic or molecular forces (‘“‘chemical” and “physical” 
forces), especially of solvents, which may render the state of 
aggregation permanent within certain limits. There is no sharp 
line to be drawn between liquids and solids, as may be seen in 
the case of a cooling gelatin solution; and although the transition 
from the liquid to the gaseous state seems abrupt, we have 
liquids of all degrees of mobility. Whether we regard a substance 
as solid or liquid will therefore depend upon the tests or criteria 
we fix for these states. 

Furthermore substances liquid when in mass, need not neces- 
sarily act so when dispersed in or adsorbed by another substance. 
Small mercury globules act like a soft “solid” metal, and col- 
loidal mercury acts like a suspensoid. Bridgman ?* has shown 


23 Z,. physik. Chem. 98, 14 (1921). 

276 J. Am. Chem. Soc. 438, 434 (1921). 

7. Hatschek, Trans. Faraday Soc. 12, 17 (1916). 
2p. W. Bridgman, J. Franklin Inst., March, 1914. 


THE STRUCTURE OF GELATIN SOLUTIONS = 75 


that water under high pressures is solid even at ordinary tem- 
peratures, and there are many reasons for believing that the 
water adsorbed by gelatin and other colloids no longer acts as a 
mobile liquid. 

In considering gel formation we cannot neglect the kinetic 
factor. The ultramicroscope shows that as particles grow smaller 
their Brownian motion increases rapidly; so that there comes a 
point where a small increase in temperature will change the gel 
into a fluid. Stirring, rubbing or other mechanical separation 
will suffice, as may readily be demonstrated ultramicroscopically 
with agar gel, from which a sliding cover glass dislodges ultra- 
microns that assume active motion. 


The Ultramicroscopic Evidence. 


Let us now consider the ultramicroscopic evidence in the case 
of gelatin solutions and jellies. 

When warm, pure gelatin solutions appear practically homo- 
geneous, but on cooling there forms a submicroscopic or amicro- 
scopic heterogeneity, depending on concentration. The largest 
particles are seen in 0.5 to 1 per cent. solutions which set to weak 
jellies and show flocks of microns and submicrons. Below 0.1 
per cent. and over 6 per cent., no ultramicrons are visible, al- 
though the polarization of the Tyndall beam proves the presence 
of amicrons. 

W. Menz”* followed ultramicroscopically the formation of 
gelatin gels. As a 0.5 per cent. gelatin solution cools, numerous 
submicrons appear and join to form flocks. Shortly before the 
submicrons appear the field becomes luminous, and the new 
phase appears suddenly in the form of tiny drops.*° 

Zsigmondy, under whose direction much of this work was done, 
states: ** “On warming, a motion on the surface of these drops 
can be seen and they either become invisibly small, or the con- 
tours gradually fade and the place where the drops were is now 
characterized for some little time by a glimmering zone. Evi- 


2 Z. physik. Chem. 66, 129 (1909); P. P. von Weimarn, Kolloid Z. 4, 133 
(1909) ; 6, 277 (1910) ; “Grundztige der Dispersoidchemie,’’ Dresden, 1911; W. 
Bachmann, Inaug. Diss. Géttingen, 1910; and von Lepkowski, Z. physik. Chem. 
75, 608-614 (1911). 

30 The converse of this is seen in the case of the digestion of partly coagulated 
egg albumen by pepsin. See J. Alexander, J. Am. Chem. Soc. 32, 680 (1910). 
' 41“The Chemistry of Colloids,” p. 226. 


76 GLUE AND GELATIN 


dence of the small diffusion in the liquid is afforded by the fact 
that on cooling, the drops may be obtained on the spot where 
they disappeared. In fact, two particles that were prevented 
from uniting by warming, may be so far restored that they may 
still unite after the cooling process has been carried out. In 
contradistinction to the case of gelatin, submicrons from crit- 
ical systems unite to form a homogeneous phase even after the 
cooling. The drops are circular, large, and have no such vari- 
ations in form as are so prominent in the case of gelatin par- 
ticles.” | 

The actual facts are probably by no means as simple as most 
of these explanations of gel formation assume. The structure 
of the dispersed phase of gels seems to be at least duplex, if not 
even more complicated, and seems to be controlled largely by an 
equilibrium between two opposing forces—(1) the attraction of 
the molecules or ultimate particles for each other, (2) the attrac- 
tion of the molecules or molecular groups for the dispersion 
medium, which is water in the case of hydrosols. Molecular 
aggregation proceeds to a certain point dependent mainly on 
temperature, pressure and chemical nature, and the primary 
particles thus formed unite to make larger aggregates or second- 
ary particles.2? The adjustment of this equilibrium takes time, 
which accounts for the fact observed by F. Stoffel ** that quickly 
chilled gelatin and slowly chilled gelatin exhibit different perme- 
ability to the same diffusing substance, but become equalized 
upon standing several days at room temperature. This slow 
annealing or hysteresis is evidently the consequence of a pro- 
eressive aggregation. 

It is only to be expected that gelatin solutions of varying 
concentration will yield gels of different interior structure, and 
therefore dry gelatins of different water absorbing capacity. 
Thus L. Arisz (loc. cit., p. 88) found that 10 and 20 per cent. 
jellies swell much more than 50 and 80 per cent. jellies, and 
W. D. Bancroft ** mentions unpublished experiments of Cartledge 
showing that 8, 16, 24 and 32 per cent. gelatin jellies, when 
dried to 96 per cent., each took up water at a different rate. 


“8See W. Mecklenberg, Mitt. K. Materialpriifungsamt. 37, 110 (1919); The 
Svedberg, Proc. Faraday Soc. (1920); Chem. Met. Eng. 24, 26 (1921). 

33 Inaug. Diss. Ziirich, 1908. 

**W. D. Bancroft, “Applied Colloid Chemistry,” p. 250. 


THE STRUCTURE OF GELATIN SOLUTIONS 77 


Gelatins made from dilute solutions are more bibulous,*® and 
swell more than those made from more concentrated solutions. 
Probably their “secondary” particles are smaller and more 
numerous, which means that they possess a greater “free” sur- 
face. 


Polariscopic Evidence. 


C. R. Smith ** has shown that at 35° gelatin in 3 g. per 100 ce. 
solution exhibits a specific rotatory power (a), of — 120.6° to 


— 123.5°, or of — 141° figured on a moisture- and ash-free basis. 
When cooled at 15° or below, (a), is found to be about — 272°, 


or — 313° on a moisture- and ash-free basis. The ratio between 
(a), at 35° and 15° is thus 2.21 to 1, and holds for the best 


grades of commercial gelatin whether made from bones, hides 
or Russian isinglass. 
Smith attributes this mutarotation to a thermo-reversible 
equilibrium 
Sol form A= Gel form B, 


which to him appears to be a bimolecular reaction, disturbed to 
some extent by some other reaction, possibly of a monomolecular 
nature, which takes place at the same time. Between 35° and 
15° both forms co-exist, and levorotation, signifying increasing 
formation of the gel form B, closely parallels increase in viscosity. 

Davis and Oakes *** find that the transition point Sol form 
A = Gel form B lies between 38° and 38.1°, and by interpolation 
fix it at 38.03°. 

M. H. Fischer (private communication) considers this the 
transition realm of water in gelatin to gelatin in water. 

C. R. Smith *’ has studied polariscopically and by jelly 
strength tests, the effect of salts and acids on the sols gel 
equilibrium. ‘They change both the final state and the velocity 
with which it is reached. Sulphates displace the equilibrium 
toward the gel side between 15° and 35° C. and the maximum 
jelly strength is rapidly reached at low temperatures. Chlo- 


*>This may be one reason why some people prefer “thin cut’’ glues, another 
reason being eaSe of solution. 

38 J. Am. Chem. Soc. 41, 185 (1919). 

36a Clarke H. Davis and Harle T. Oakes, J. Am. Chem. Soc. 44, 464 (1922). 

87 Private communication of unpublished work. 


| 
| 
| 


78 GLUE AND GELATIN 


rides, bromides, and iodides lower the viscosity of gelatin and 
shift the equilibrium toward the sol side. Salts and even weak 
acids do not, full strength being reached if a low temperature 
(about 5° C.) is maintained for a sufficient time. Therefore 
equilibrium rotation with sulphates is larger than with pure 
water, whereas with iodides it is less. 

The facts resemble those met with in the dynamic allotropy 
of sulphur,?* and indicate the formation, with decreasing tem- 
perature, of larger molecular aggregates, which, however, need not 
be chemical compounds in the ordinary acceptation of the term. 
It is interesting also to note that L. Arisz®® found that the 
Tyndall phenomenon in gelatin increases in intensity with fall- 
ing temperature, and is also dependent on the previous history 
of the gelatin, which controls the size of its particles. 

In the case of colloidal metals, the attraction of the metal 
molecules for each other is so powerful that unless an adsorbed 
layer of protector or ions intervenes, the aqueous films are 
squeezed out, and there results a more or less water-free metal 
sponge, bathed in practically metal-free water. With most 
oxides and sulphides the mutual attraction of the molecules is 
not so great, probably because the main attractive forces have 
been satisfied in the formation of the chemical compounds in 
question, or the attraction for water is greater; and therefore 
the tendency to gel formation is greater. G. Varga * estimated 
that particles of stannic acid gel 12.7 wu in diameter contain 
only 1g their volume of massive stannic oxide, the remaining 
% being mainly water. The micellular complexes even of dry 
gelatin, contain a very large amount of water, the higher cae 
holding most. 


Complex Structure of Gelatin. 


With gelatin we have a mixture of large and highly polar 
molecules containing NH, and COOH groups, which probably 
form adsorption +! aggregates that constitute the primary parti- 
cles of gelatin. These primary particles have an interior struc- 
ture with extremely fine capillaries, because the residual attrac- 


33 WwW. E. S. Turner, “Molecular Aggregation,” p. 92. 

39. Arisz, Kolloidchem. Beihefte 7, 1 (1915). 

* Kolloidchem. Beihefte 11, 1 (1919). 

41'The larger and more complex the reacting masses the more “physical” 
rather than “chemical” does the reaction appear to be. 


THE STRUCTURE OF GELATIN SOLUTIONS 79 


tions of the constituent molecules are relatively weak and are 
unable to displace the adsorbed water films which make the 
colloid hydrous and hydrophile. It is furthermore probable that 
the “fluid” surrounding the primary particles is not pure water, 
but is an aqueous solution containing a larger portion of the 
more soluble constituents or those with smallest molecules.*? 

The primary particles form secondary groups in this “fluid,” 
which fills the larger capillary spaces produced thereby. In- 
crease in temperature, or the addition of certain salts (e.g. CaCl, 
NaNO,), cause dispersion again into smaller or primary particles 
which is mainly reversible; but continued or high temperature 
or the presence of much acid and especially alkali, seems to 
attack the primary groups and bring about what we term hydrol- 
ysis, a splitting up of the adsorption complexes into their con- 
stituent polypeptides, and ultimately degeneration into amino- 
acids.** 

S. E. Sheppard and F. A. Elliott 44 see no need of postulating a 
sub-microscopic but supermolecular structure in gelatin, at- 
tributing any “structure” to an environment impress. 

Since P. W. Bridgman by mere pressure produced a new black 
allotropic form of phosphorus, and demonstrated that the same 
very high pressures produced in water molecular aggregates 
which persisted for days, it is reasonable to assume the existence 
of a sub-microscopic but supermolecular structure in gelatin, at. 
the surfaces of which great compression is known to occur. 

In a technical paper on “Colloidal Fuels” S. E. Sheppard 4# 
made a suggestion as to the emulsoid colloid state, which was 
developed more fully in a letter to “Nature.” 44° The essential 
feature of the hypothesis put forward was that the micelles, or 
plurimolecular units of such colloid systems, are formed, and 
their growth and aggregation determined by “the orientation of 


“ Segregations of this kind are common, especially in soaps and alloys. See 
Jerome Alexander, “Colloidal State in Metals and Alloys,’ Trans. Am. Inst. Min. 
and Met. Eng., Vol. 64 (1920); Chem. Met. Eng., January, 1922. 

# W. Mecklenberg attributes the differences in the nature of stannic acid gels 
to variations in the size of their primary particles. 

45. Am. Chem. Soc. 44, 873 (1922). F 

s4a, J, Ind. Hng. Chem, 13, 37 (1921). These remarks are taken from an 
advance copy of a paper entitled ‘The Interfacial Tension between Gelatin and 
Toluol,” by 8. E. Sheppard and §..8S. Sweet, to appear in J. Am. Chem. Soc. My 
thanks are due to Dr. Sheppard for his courtesy. 

44b “The Nature of the Hmulsoid Colloid State,’ Nature, March 17 (1921), 
| a oe 


80 GLUE AND GELATIN 


definite atom groups, entirely in the sense of the theory of 
molecular orientation due to structure proposed for surface and 
interfacial tension phenomena by W. B. Hardy,**¢ W. Harkins 444 
and J. Langmuir.**¢ 

“The genesis of a micelle, as plurimolecular unit of a colloid 
system, may be regarded as a consequence of equilibrium, usually 
incomplete, between homochemical solution forces and hetero- 
chemical forces, the former tending to dissociate and decompose 
the chemical molecule, the latter resisting decomposition. In 
the case of proteins the most probable general type of linkage, 
according to R. H. A. Plimmer ** is of the form 


NH,.CHR.CO. (NH.CHR.CO),.NH.CHR 


| 
COOH 


when n refers to the degree of polypeptide condensation and R 

is an alkyl or other substituent group. On the hypothesis sug- 

gested here we may, imperfectly, represent the redistribution of | 

this in the presence of water for the polypeptide chain by © 
Aqueous 


Zone HOH HOH 
fe Fe o™ 
wf Z f aN 7 Fa < 
O H O H O 
C ——_——_ H C N C 
tae a fi 
wwe ws 
CHR (CHR) CHR (CHR) CHR 
Lipoid 
Zone pe 


In this the arrows indicate the direction of an imagined plane 
or intra-molecular interface 7 separating the hydrophile groups 


5 2 which are consolute with water (in virtue of residual 
affinities tending to complete the amino and carboxyl groups), 
from the hydrophobe or hydrocarbon groups—CHR. Not only 
in one and the same protein molecule, but also to a variable 
extent between molecules, we may admit that this primary 
orientation leads to mutual attraction between water-soluble and 

44c Proc. Roy. Soc. 81A, 610 (1912). 

44d J, Am. Chem. Soc. 39, 354 and 541 (1917). 


ue J, Am. Chem. Soc. 38, 2221 (1916): . 
44f “Chemical Constitution of the Proteins,’ II. p. 2. 





THE STRUCTURE OF GELATIN SOLUTIONS 81 


water-insoluble groups respectively. Without any actual cleav- 
age of the molecule, we have orientation and a stratichemical 
field of force which is of a similar character, in essence, to crys- 
tallization, but results in incomplete instead of complete equi- 
librium. The hydrocarbon or lipoid atom groups will approach 
the fluid on the solid state according to molecular weights and 
constitution; hence the system may be likened, in one aspect, 
to a sub-molecular emulsion, the lipoid groups tending to form 
interconnected sheets of atom-groups necessarily permeable to 
water and water solutes, although mechanically developing a 
stress resisting rupture in virtue of the fields of attraction and 
repulsion induced. The micelles are the smallest plurimolecular 
_ units thus built up. 

“The following brief survey indicates the present status of the 
question. Somewhat destructive criticism of the foregoing 
hypothesis by J. W. McBain ‘#2 was shortly followed, in the 
same journal **" by N. K. Adam’s observations on mono-molecu- 
lar films of palmitic acid on water and aqueous alkali solutions. 
They confirmed the theory of the orientation of soap molecules 
in surfaces and micelles, suggested by Harkins, Davies and 
Clark.*4 Further, the structure of the soap micelle proposed 
by Adams was quite in accord with Sheppard’s suggestion that 
orientation determined the growth of the micelle. More recently, 
J. Loeb *4i has explained the stability of protein solutions and 
the difference between gel formation and precipitation by ref- 
erence to an orientation hypothesis of the protein molecule. 
Loeb’s “watery” groups and “oily” groups correspond respec- 
tively to the “hydrophile” and “hydrophobe” or “hydrocarbon”’ 
groups of Sheppard’s note. Finally, it may be noted that E. J. 
Witzemann, in an interesting paper *** considers that orientation 
at surfaces, as shown by soaps, is of less importance for proteins 
and polysaccharides. Generally, however, his argument sup- 
ports a chemical view of the biocolloids. 

“Whatever the increased consideration gained for the hypothe- 
sis by these contributions, it remains actually a working hypothe- 
sis, to be tested by definite consequences capable of experimental 

“zg Nature, loc..cit., p. 74. 

4h [bid., April 28, 1921. 

“41 Loc. cit. 


44j “Proteins and the Theory of Colloid Behavior,” p. 283 (1922). 
“4k J, Phys. Chem, 26, 201 (1922). 


82 GLUE AND GELATIN 


verification. These consequences reach in two directions,—on 
’ the one hand, the behavior of emulsoid colloids to their thermo- 
dynamic environment,*! on the other, fundamental chemical 
changes (oxidation-reductions, substitution, etc.). On the first 
count, the surface and interfacial tensions of emulsoid colloids . 
are of particular interest. It is known that on shaking weak 
solutions of gelatin, with immiscible solvents such as benzole, 
gasolene, toluene, etc., gelatin, still considerably hydrated, tends 
to be thrown out and aggregated as an interfacial layer.**™ 

“Tt appeared desirable to investigate this more fully, in par- 
ticular as a function of hydrogen ion concentration. The prop- 
erties of gelatin as an emulsifying agent for kerosene have been 
studied by H. N. Holmes and W. C. Child “ in relation to. 
(a) the surface tension of the gel-oil interface, (b) determination 
of whether or not gelatin is adsorbed to form a concentration 
layer around the oil droplets and (c) viscosity of the solution. 
The present investigation, while not at variance with their 
results, shows that in such studies the hydrogen-ion concentration 
may be a determining factor. This is to be expected, but the 
relation of the property to p,, in the present case is somewhat 


different from those instanced by Loeb and others. It will be 
remembered that in the cell protoplasm we have a complex 
lipoid-protein interface, so that the property in question is 
physiologically important, as also industrially, in relation to 
certain processes for preparing glue and gelatin.” 

Although the original polypeptides or amino-acids composing 
gelatin may have a limited power to crystallize, in mixtures we — 
are confronted with what appears to be a general tendency on 
the part of substances of different crystallization speed to inter- 
fere with each others normal crystallization. This tendency 
seems to be due to the fact that in crystallizing, all substances 
must pass into or through the colloidal zone where they are apt 
to be adsorbed by larger particles; it is exhibited by glasses 
and metals, by soaps, and by mixtures of fatty acids, and tends 
to keep the mixture in a state of fine aggregation. 

Some substances, assuming an iso-colloidal state, are able to 

441 On the orientation theory, their electromagnetic environment. | 

44m Winkelblech, Zeit. f. angew Chem. 13, 1753 (1900); ef. also W. Bancroft, 


“Applied Colloid Chemistry,” p. 260. 
4in J, Am. Chem. Soc. 42, 2049 (1920). 


THE STRUCTURE OF GELATIN SOLUTIONS ~ 88 


interfere with their own crystallization (auto-protection) .*® 
‘Hardy found that 5-dimethylaminoanilo-3, 4-diphenylcyclo-1, 2 
dione, upop cooling its solutions in organic solvents, gives gels 
which gradually become crystalline. (See also p. 49.) Fre- 
quently substances whose crystallization is interfered with, as- 
sume the form of tiny globulites, as is the case with lactose for 
example. Concentrated solutions of sucrose crystallize with 
difficulty and act somewhat like a “glue.” ‘The tendency toward 
crystallization is markedly inhibited by colloidal protectors such 
as glue, gum arabic, etc.*® 

From the evidence at present available, the following picture 
may be drawn of the formation of a gelatin gel from any ordi- 
nary warm solution of gelatin: 

(1) The hot solution contains adsorption complexes of poly- 
peptides (“gelatin molecules”) which possess residual unsatisfied 
free fields of force, and a powerful idioattraction. ‘These com- 
plexes with their adsorbed ions, are dispersed in a “fluid” con- 
taining, in still finer dispersion, hydrolysis products of the origi- 
nal gelatin complexes, and ions from the dissociation of salts 
(including “salts” of gelatin), acids, or alkalis; and also contain- 
ing the undissociated salts, etc. 

(2) As the temperature drops, and thermal agitation dimin- 

ishes, the gelatin “‘molecules” begin to aggregate. As these 
ageregations increase in size, their Brownian motion diminishes, 
until they finally form clustered, almost motionless, masses, 
which, if the concentration is not too small, are on all sides 
within the range of each other’s molecular attraction. That is, 
although they are actually separated by adsorbed aqueous films, 
they practically “link arms” to form what D. Jordan Lloyd 
calls a continuous solid phase. 
_ (3) The size of these molecular aggregations and the size of 
their tiny pores, will vary with the nature of what is adsorbed 
at their interfaces; that is, will vary with the nature of the “im- 
purities” present. Speed of chilling and tempering also exert a 
syneretic influence, just as they do.with metals. Viscosity and 
jelly strength will vary with particle size, there being a zone 
of maximum colloidal effect.*% 


46See eg. W. B. Hardy, Proc. Roy. Soc. London (A), 87, 29 (1913); J. 
Alexander, J. Ind. € Eng. Chem. (19238). 

46 J, Alexander, J..Soc. Chem. Ind. 28, 280 (1909). 

46a See Jerome Alexander, J. Am. Chem. Soc. 484 (1921). 


84 GLUE AND GELATIN 


(4) The remaining “fluid” or dispersing phase, which sur-. 
rounds the molecular complexes, will be in a state of kinetic 
equilibrium with the molecular complexes, so far as concerns 
particles or ions diffusible into the pores of the latter. This 
dispersing phase will contain most of the water, the highly solu- 
ble products, and those ions which are too large to enter the pores 
of the “solid” phase or which are unadsorbed.** 

(5) The adsorption of ions (especially hydrogen or hydroxyl 
ions) tends up to a certain point to separate the gelatin “mole- 
cules” constituting the molecular groups ** and thus enables the 
groups to take up more water. Beyond this point, “salting-out,” 
sol formation and hydrolysis predominate.*** ‘The same effect 
may also be accounted for on the basis of the Donnan theory.**> 

In a heterogeneous mixture of complex groups such as are 
found in gelatin solutions or jellies, it is very unlikely that there 
is any definite arrangement of molecules into threads, chains, or 
strings. Since molecular groups adjoin each other in every direc- 
tion, it is only natural that microscopic or even ultramicroscopic 
examination reveals what seem to be spheres, which, according 
to the focus of the instrument, may appear to form elongated 
groupings. ‘Those familiar with the limitations of optical instru- 
ments will understand that these apparitions are only diffraction 
images of irresolvable particles. In fact, Scherrer has shown 
with the X ray spectrometer that gelatin is truly amorphous.*® 

Comparing gel structure to a “pile of shot,” °° “anastomosing 
threads,” ®+ “thread-like crystals,” °* “streptococcal threads,” *8 
hardly gives one a correct picture; for while the imagination may 
isolate such groupings from the mixture of molecular groups, 
they have no real existence. It is true that the polar nature of 
the molecules may tend to produce some kind of orientation, 
and that some chain-like structures may be formed; but in gen- 
eral the tendency does not establish itself, and the incidental 


47 Some (eg. H. R. Procter, J. A. Wilson and J. Loeb) believe that gelatin 
forms definite salts and that a Donnan membrane equilibrium exists. 

48Tolman and Stearns, J. Am. Chem. Soc. 40, 264 (1918). 

48a A, Kuhn, Kolloidchem. Beihefte 14 (1921). 

48b J. Loeb, ‘Proteins and the Theory of Colloid Behavior.” 

49P, Scherrer, Nach. Ges. Wiss. Géttingen, 1918. 

50 Bradford, Biochem. J. 12, 382 (1918). 

3 T. B. Robertson, “The Physical Chemistry of the Proteins,” 1918, p. 302. 

52 W. Moeller, Kolloid Z. 23, 11 (1918). 

8 R, H. Bogue, Chem. Met. Eng. 23, 61 (1920). 


THE STRUCTURE OF GELATIN SOLUTIONS — 85 


‘formation of chains or threads is not an essential of gel forma- 
tion. Jelly formation occurs even in emulsions where there is 
no evidence of chain structure. In the welter of conflicting 
attractive forces and closely packed molecules, the weak residual 
attractive forces remain unsatisfied, so that there may be a state 
of stress which is a cause of elasticity of the jelly. With very 
‘dilute solutions of gelatin, however, polar grouping in chains 
probably takes place to a considerable extent, as ultramicro- 
graphs of such solutions would indicate.** 

D. Jordan Lloyd ** has followed the action of hydrochloric 
acid, sodium hydroxide and sodium chloride on the gelling power 
of gelatin purified by dialysis at the isoelectric point. Taking 
the minimum quantity of gelatin required to produce a gel after 
standing at 15° for 48 hours, she found the minimum concen- 
tration of pure gelatin to be 0.8 percent. Hydrochloric acid 
lessens the gelling power, showing maximum reduction at P 5, 2-3, 


and again at higher acidity than p,, 0.7. Sodium hydroxide 
causes slight decrease between p,, 10-12, and above this prevents 


gelatinization. Though no simple relation between sodium 
chloride content and gel power was evident, neutral salts oppose 
the action of H ions. 

The fact that gelatin may be altered by “molecular bombard- 
ment” was shown by E. Miihlenstein,®> who exposed a gelatin 
layer 46 u thick to X rays from polonium, and found, after soak- 
ing in water and drying, a permanent depression of about 22 u in 
the exposed portion of the gelatin. As the depression is invisible 
prior to the soaking, it is not purely mechanical, but is evidently 
due to some change in molecular structure. 

A. Tian ** found that wave lengths of quartz-mercury ultra- 
violet light of 3,000 A which coagulate albumin, did not affect 
dry gelatin and only fluidified the jelly. 

Gelatin jellies, upon being strained, show double refraction, 
an evidence of anisotropic structure. 


54 J. S. van der Lingen, J. Franklin Inst. 191, 651 (1921), finds that the 
pseudoisotropic layers in such anisotropic liquids as p-azoxyanisole, p-azoxy- 
phenetol, anisaldazine, and ammonium p-cyanobenzalaminocinnamate (Stumpf’s 
ester), do not possess a space-lattice, and show no evidence of being micro- 
crystalline. 

54a Biochem. J. 16, 530-45 (1922). 

5 Arch. sci. phys. nat. 2, 423 (1920). 

58 Compt. rend. 151, 219 (1910). 


86 GLUE AND GELATIN 


P. W. Bridgman’ subjected gelatin jelly to a pressure of. 
9,000 kilos per sq. cm., and found no visible change except 
that the gelatin was cracked into rather large lumps, doubtless 
because at this pressure water freezes into one of the four 
varieties of pressure-ice, making the gelatin jelly so rigid that it 
could not accommodate itself to the shape of the containing 
vessel. 

The addition of alcohol to gelatin solutions dehydrates the 
micellular groups, converting the “emulsoid” gelatin into a “sus- 
pensoid”’ opalescent solution which shows ultramicrons.°® W. 
O. Fenn °° has studied the effect of electrolytes upon this change. 

57 Private communication. 

58 See e.g. O. Scarpa, Kolloid Z. 15, 8 (1914). 


588 Proc. Nat. Acad. Sci, 2, 5384 (1916); J. Biol. Chem. 22, 279, 34, 141 and 
415 (1918). 


Chapter 6. 


The Influence of Various Factors on the Swelling 
of Gelatin. 


Many factors influence the amount of water absorbed by dry 

gelatin, and also the speed with which the water is taken up. 
Among these are: The ratio of the free surface area to the 
volume; the hydrogen 10n concentration; * the temperature of 
the system; * the elastic modulus of the gelatin; * the ratio of the 
mass of the gelatin in the system to the mass of dissolved electro- 
lyte;® the previous history of the gelatin, upon which depends 
its internal structure; *® and the effect of unclassified substances 
like urea, pyridine and the amines. 
_ When gelatin swells in water the volume of the swollen gelatin 
is less than the combined volumes of the original gelatin and the 
absorbed water.’ Heat is developed by the absorption of water,® 
indicating that there is a compression or condensation of the 
water coincident. with its entrance within the capillary and 
molecular spaces of the gelatin. This “heat of swelling” is 
analogous to the heat developed when moisture is absorbed by 
superdried peas, starch or dextrin. 

H. G. Bennett ® attributes the heat liberated by relEAe gelatin 


1’, Hofmeister, Arch. erp. Path. Pharm. 27, 395 (1890) ; Wo. Pauli, P/fliiger’s 
Arch. 67, 219 (1897) ; Spiro, ““van Bemmelen Festschrift,’ 1910, p. 261; M. H. 
Fischer, ‘““‘Das Oedem,’”’ Dresden, 1910. 

2Chiari, Biochem. Z. $8, 167 (1911); H. R. Procter, J. Chem. Soc. 105, 313 
(1914) ; J. Loeb, J. Gen. Physiol. 1, 41 (1918). 

’ Procter and Burton, J. Soc. Chem. Ind. 35, 404. 

4Procter and Wilson, J. Chem. Soc. 109, 307 (1916). 

5D, Jordan Lloyd, Biochem. J. 14, 149 (1920). This seems to be an illus- 
tration of the Donnan equilibrium. J. A. 

6éWw. R. Hardy, Proc. Roy. Soc. 66, 95 (1900); Arisz, Kolloidchem. Beihefte 
7%, 1 (1915) ; see also W. D. Bancroft, ‘“‘Applied Colloid Chemistry,” p. 251, 1921. 

7G. Quincke, Arch. f. d. ges. Physiol. 8, 332 (1870). 

8H. Wiedemann and C. Ltideking, Wied. Ann. 25, 145 (1885). See also HE. 
Hatschek, “An Introduction to the Physics and Chemistry of Colloids,’ London, 
19138, p. 55. 

®°H. G. Bennett, ‘‘Animal Proteins,” p. 204, 1921. 


87 


~/ 


88 GLUE AND GELATIN 


(5.7 calories per gram of gelatin) to the compression of the 
absorbed water, and from the LeChatelier theorem predicts what 
is actually found—that gelatin swells best in cold water. ‘The 
faet of water compression determines the rigidity of the gel, 
and the changes in this compression of the continuous phase ?° 
determines the surface tension resultant which hinders swelling, 
and which is one of the two main factors fixing both the rate 
at which gelatin swells in water, and the final volume attained by 
the gel” (p. 209). 

The stiffening, shrinking and “salting out” action of sulphates, 
tartrates, etc., Bennett considers to be examples of “lyotrope” 
compression, while the contrary effect is exhibited by iodides, 
thiocyanates and urea, which may entirely inhibit gelatinization. 
In this latter respect Bennett’s “lyotrope series” is marred, for 
calclum, magnesium and zinc chlorides, as well as sodium and 
calcium nitrates also inhibit gelatinization. 

Increase in temperature causes an increased heat of swelling, 
which leads Wilson '” to the conclusion that the heat is due, not 
to swelling, but to chemical combination between the gelatin 
and a small portion of the absorbed water. The more likely 
explanation is that in warmer water gelatin swells more rapidly, 


. thus producing more heat per unit of time. For as Hofmeister ™ 


observed, the swelling of gelatin plates has a higher initial 
velocity, after which it proceeds more slowly to a maximum. 
Furthermore at higher temperatures the gel particles probably 
undergo a further dispersion resulting in more free surface. 

Thus Arisz** observed the following differences in water ab- 
sorption by gelatin with variation in temperature, the figures | 
given representing the weight of one gram of swollen gelatin at 
the time indicated. 


1 Bennett here means the water is the continuous phase. But in a jelly it is 
probable that the dispersed phase is also ‘continuous.’ ‘Bancroft gives as an 
illustration of such condition a roll of wire fencing standing in the air—the 
wire is continuous but so also is the air. Strictly speaking, all matter consists 
of discrete particles. J. A. 

1 The other factor, according to Bennett, is the ‘“lyotrope’ (Hofmeister 
series) effect of salts, etc., upon the compressibility of water. J. A. 

2 J, A. Wilson, 8d Report on Colloid Chemistry, British Association A. S. 
(1920), p. 51. : 

13, Hofmeister, Arch. f. Exper. Path. und Pharm. 27, 395 (1890) ; 28, 210 
(1891). 

1440, Arisz, Kolloidchem. Beihefte 7, 49 (1915). 


THE SWELLING OF GELATIN 89 


Temperature 1st day 2ndday  38rdday 
2° 10-— 10 10 — 
Les 10 10 10 
20° 16 18 19 
267 33 40 46 
30° 35 within the first few hours followed 


by solution. 


The figures tabulated are approximate, as Arisz’ results are 
given in curves which show that most of the water is taken up 
within a few hours. 

Hofmeister and Wo. Ostwald have also pointed out the influ- 
ence of the shape of the piece of gelatin on the degree of swell- 
ing. Thin sheets not only swell more rapidly, but they also swell 
more than thick sheets.1° The amount of water absorbed by a 
piece of gelatin depends very materially upon the previous his- 
tory of the gelatin which influences its internal structure. (See 
p. 83, Chapter 5). Procter dried out (presumably at low tem- 
peratures) three jellies containing respectively 5, 10 and 20 per 
cent. of gelatin. Upon soaking these in cold water for seven days 
he found that they absorbed respectively 14.6, 7.7, and 5.8 times 
their weight of water. 

Arisz *® gives curves showing the influence of the age of a block 
of gelatin jelly upon its capacity to absorb water. Freshly 
prepared jellies absorb most water, which is an indication of the 
progressive aggregation of the gelatin particles in aging jellies 
(syneresis), with a concomitant diminution of free surface. 
Curiously enough, Arisz found, contrary to expectation, that a 
block of gelatin jelly swollen at 10° loses water when warmed to 
20°; and a jelly first swollen at 20° actually swells faster when 
the temperature is reduced to 10°. These variations in water 
absorption are influenced by the previous gel history. Appar- 
ently two opposing factors are at work: Ist, heat tends to 
produce finer dispersion with greater water absorption; but, 
2nd, the heat seems to relax the attraction of the gelatin for the 


water, probably because of increased kinetic activity. 

1% Therefore a ground glue or gelatin would appear to absorb more water 
than the same product in flake form. Incidently this indicates a source of 
error in grading on the basis of ‘‘water-absorption,’ and a possible reason why 
some users who test on this basis, prefer thin cut flakes of glue or gelatin. 

4¢ Hoc. cit., p. 57. 

17 For further details and experiments on the intermittent swelling and drying 
of gelatin jellies, the reader is referred to the original paper of Arisz. * See also 
A. G. Brotman, J. Soc. Leather Trades Chem. 5, 226 (1921). 


90 _ GLUE AND GELATIN 


In general gelatin swells more in the solution of any acid or 
any alkali than it does in pure water.‘® With very minute 
quantities of acid there is a slight diminution of swelling, the 


minimum with HCl being at a concentration of sai’ after 





n 
90.9 
swelling beyond that which occurs in pure water. Wo. Ostwald’s 
results show that both with HCl and KOH the swelling reaches 


which the curve rapidly rises, so that at there is already a | 


a maximum about aaF after which it slowly diminishes. 


According to Fischer (loc. cit., p. 29), if two like gelatin discs 
are simultaneously immersed, the one in pure water and the other | 
in x HCl, the superior degree of swelling of the latter is plainly 
visible at the end of six hours, and is still more marked after a 
day or two. Then the disc in the pure water is still somewhat 
yellow and cloudy, whereas the gelatin in the dilute acid is 
swollen so clear and hyaline, that it can hardly be seen at the 
bottom of the dish. 


Lyotrope or Hofmeister Series. 


Wo. Ostwald inclined to the belief that the swelling was exclu- 
sively a function of the H-ion concentration of the acid solution, 
whereas Fischer believes that it is determined by the concen- 
tration of the H-ions minus the effect of the particular anion 
concerned. M. H. Fischer’s experiments show that the order of 
acids in increasing the swelling of gelatin is as follows: 

HCl>HNO,>CH,COOH > H,S80,>H,BO, 

“The position of the ‘weak’ acetic acid between the ‘strong’ 
nitric and sulphuric acids (which two are about equally disso- 
ciated, and yield a higher concentration of hydrogen ions than 
the equinormal acetic acid) is by itself an argument against the 
explanation which considers only the concentration of hydrogen 
ions.”’ 7° 

1K, Spiro, Beitrdége zur chem. Physiol. 5, 276 (1904) ; Wolfgang Ostwald, 
Pfliiger’s Archiv. 180, 563 (1905) ; M. H. Fischer, ‘“Hdema,” New York, 1910. 

19 Fischer, loc. cit., p. 31.- These series of anions and cations are known as 


the lyotrope (or solution changing) series. They are also termed the Hofmeister 
series in honor of their discoverer. 


THE SWELLING OF GELATIN 91 


Gelatin is so sensitive to the presence of acid, that highly puri- 
fied gelatin will swell less in conductivity water than in ordinary 
distilled water which contains CO,.”° 

With alkalis there is no initial decrease in swelling. Their 
order in increasing swelling is as follows: 


KOH>NaOH>Ca(OH),>NH,OH. 


Since at the concentrations employed, the dissociation of the 
first three of these alkalis is about the same, Fischer concludes 
that the swelling of gelatin in various alkalis 1s dependent upon 
the OH-ion concentration minus the effect of the cation. Thus 
calcium is more active in inhibiting swelling than is sodium, while 
potassium permits the greatest swelling.”* 

It is well known biologically that bivalent ions counteract 
the injurious effect of monovalent ions, which often act as poisons. 
The antitoxic action of polyvalent ions has been demonstrated 
by Jacques Loeb on the fertilized eggs of fundulus herocltus, a 
small fish, by R. 8. Lillie on the larval forms of arenicola, 
a sea annelid, and by Wo. Ostwald on gammarus pulex, the 
sand flea. Many animals which live in sea-water are killed by 
sodium chloride solutions isotonic with sea-water. Ostwald has 
also shown that the swelling of gelatin is much more power- 
fully depressed by polyvalent ions than by monovalent ions 
(Mg<Ca<Ba<Sr<Cu<Fe), and M. H. Fischer has had like 
results with fibrin. It therefore seems that the “impurities” of 
sea-water keep the biocolloids at a certain optimum degree of 
swelling or turgidity. 

R. S. Bracewell 2? believes that the amount of acid adsorbed 
by proteins is determined mainly by their content of the two 
diamino acids, lysine and arginine; and his experimental data 
show that the acid adsorbed per gram of protein (gelatin, fibrin, 
casein, gliadin, edestin) is roughly proportional to the number of 
free NH, groups per gram of protein. , 

Tolman and Stearn ?* attribute the swelling of gelatin, fibrin 

20T. Oryng, Kolloid. Z. 17, 14 (1915). Conductivity water has py, =7, while 


ordinary distilled water has py=95.5 because of dissolved CO: Wo. Pauli 


first pointed this out about 1905. 

21 This difference may be of importance in the functioning of muscle, especially 
heart-muscle. C. R. Smith (J. Am. Chem. Soc. 43, 1860 [1921]) gives figures 
on the swelling of ash-free gelatin in various alkalis. : 

22, J, Am. Chem. Soc. 41, 1511 (1919). 

*R,. C. Tolman and A. E. Stern, J. Am. Chem. Soc. 40, 264 (1918). 


92 GLUE AND GELATIN 


and similar colloids in acids and alkalis to the selective adsorp- 
tion of H or OH ions respectively at the surface of pores or 
pockets within the gel. Owing to electrostatic repulsion these 
pores increase in size, the increase being accompanied by imbibi- 
tion of the solution. When a neutral salt is added to the solu- 
tion, its ions arrange themselves in such a way as to neutralize 
the original electrostatic repulsion, thus producing shrinking. 
The fact that salts with polyvalent ions are most effective in 
producing dehydration is consequent upon the superior power of 
the latter to neutralize the existing electric field, although they 
take up no more room than monovalent ions. 

Gelatin swells somewhat less in a solution of dilute alkali than 
it does in an equinormal solution of acid. Fischer believes that 
this is due to the depressing action on swelling of a salt formed 
by the alkali in the gelatin which is generally acid in reaction. 
The results of C. R. Smith support this view. He believes that 
they drive back the ionization of the acid.2* Let us then con- 
sider the effect of salts on the swelling of gelatin. 3 

Neutral salts possess the power of inhibiting, to variable extent, 
the degree of swelling of gelatin in acids or alkalis. In reducing 
swelling the initial small percentages of salts have a relatively 
more potent influence. 

The effect of salts in depressing swelling seems to be for the 
most part the result of the combined action of its anions and 
cations. The following series shows the relative effectiveness 
in producing a depression of swelling in acids or alkalis: 


For cations 
Fe(ic) >Cu(ic) >Sr>Ba>Ca>Mg>NH,>Na>K.2# 
For anions 
citrate >tartrate > phosphate >SO,>acetate> 
I>CNS>NO,>Br>Cl. 
Thus according to Bechhold ** 0.78 gram gelatin in 100 ce. of 
0.05 n HCl swelled until it weighed 14.61 grams. In the pres- 


ence of 5 Potassium citrate it weighed only 2.84 grams, and in 


4 J. Am, Chem. Soc. 43, 1350 (1921). 

*4a M. H. Fischer (private communication) believes that the position of NH, 
is uncertain, and that it probably is the last member of the series. 

°° H. Bechhold, “Colloids in Biology and Medicine” (J. G. M. Bullowa’s trans- 
lation), 1920. 


THE SWELLING OF GELATIN 93 


the presence of > KCl it weighed about 7 grams. Bancroft *¢ 


points out that a large part of the decrease in swelling may be 
due to the diminished H-ion concentration of the solution, and 
not exclusively to the citrate ion. 

In determining the H-ion concentration of gelatin colori- 
metrically, use may be made of the well-known series of indi- 
eators described by Clark, Lubs and Acree.?* The limitations of 
such indicators must be borne in mind. 


Donnan’s Theory. 


HH. R. Procter 2* gives the following abstract of the theory of 
the swelling of gelatin in acids, evolved by himself and his pupils, 
chief among whom is J. A. Wilson: ”° 

“In equilibrium between a jelly and its external solution not 
only must all osmotic pressures be equally balanced, but as has 
been shown by Donnan,’? the electro-chemical condition must be 
fulfilled that the products of any pair of diffusible anions and 
cations common to both phases, must be equal. Thus with gela- 
tin chloride and free acid the chloridions multiplied by the hy- 
drions must be equal in the jelly and the external acid.*? On 
the other hand, the osmotic pressures depend not on the products 
but simply on the swm of diffusible particles present. In the 
external acid the numbers of hydrions and chloridions are obvi- 
ously equal, while in the jelly the chloridion of the gelatin 
chloride is added to the equal hydrion and chloridion concentra- 
tion of the free acid present, thus making the final concentrations 
of these ions in the jelly unequal. 

“Now, as the sum of two unequal factors is always greater 
than that of two equals giving the same product, or, geometrically 
the perimeter of a square is always less than that of any other 

2W. D. Bancroft, ‘Applied Colloid Chemistry,” p. 254. 

27 J. Am. Chem. Soc. 41, 1190 (1919). 

23 Hirst Report on Colloid Chemistry, Brit. Assoc. Adv. Sci. (1917), p. 8. 

27H. R. Procter and J. A. Wilson, J. Chem. Soc. 109, 305 (1916). M. H. 
Fischer (private communication) points out that rubber swells in benzol, 
nitrocellulose in ether-alcohol, and soaps in many organic solvents, although in 


such cases the existence of a Donnan equilibrium is precluded, for there is no 
dissociation. J. A. 


80 Z. Hlektrochem. 17, 572 (1917) ; Donnan and Harris, Trans. Chem. Soc. 99, 
1575 (1911). 


31 This is known as a “Donnan equilibrium.” J. A. 


94 GLUE AND GELATIN 


rectangle of equal area, and as the sides represent the osmotic 
pressure, while the area represents the product, it is clear that 
the two inequalities cannot at once be completely fulfilled, but 
in electro-chemical equilibrium the osmotic pressure must be 
in excess and the jelly must tend to swell unlimitedly and finally 
to dissolve. That it does not do so is a consequence of its col- 
loid nature, which depends upon cohesive attractions drawing 
the colloid particles together to polymerized masses or to a 
continuous network, and which consequently opposes swelling 
and solution, while the diffusible ions are held to the colloid ions 
by electro-chemical attractions, and, as they cannot escape from 
the jelly, tend to drag it apart and dilute it by absorption of the 
external acid, from which they expel a part of its acid concen- 
tration.” 

“The equilibrium is therefore a very complex one, but finally 
depends on the excess of internal osmotic pressure being bal- 
anced against the internal attraction or cohesion of the colloid 
particles, both ions and molecules. For mathematical discussion 
the reader must be referred to original papers by Procter and 
his pupils. It will, however, be obvious that as the external 
solution becomes more concentrated, the proportion of absorbed 
acid (or salt) is increased, while that of gelatin chloride is 
limited to the quantity of gelatin present. ‘The difference of 
concentration of hydrion and chloridion in the jelly is therefore 
diminished, and it contracts under the influence of its own in- 
ternal attractions. 

“Precisely similar considerations apply to the action of alkalis 
on gelatin. Jonizable salts are formed by combination of the 
base with the carboxyl group of the proteid, and the osmotic 
equilibrium is with the cation and OH instead of with the anion — 
and H. Neutral gelatin, as an amphoteric body, of course 
ionizes to a limited extent with water alone, and its dissociation 
constants are of the same order of quantity as those of the 
water with which it is in equilibrium. It. is, however, slightly 
stronger as a base than as an acid, and consequently its neutral 
point of minimum swelling is slightly on the alkaline side. This 


82 The distinction between this view and that of Tolman and Stearn, J. Am. 
Chem. Soc. 40, 264 (1918), is rather finer than claimed by J. A. and W. H. 
Wilson, J. Am. Chem. Soc. 40, 8S6 (1918). J. A. 


THE SWELLING OF GELATIN ' 95 


has important bearings on manufacturing practice,** the greatest 
flaccidity of the raw skin, which is required for the softest 
leather, being obtained in weakly alkaline liquids. 

“Tt has been pointed out by Donnan ** that in consequence of 
the unequal distribution of positive and negative diffusible ions 
which has just been described, the surface of an acid or alkaline 
jelly in equilibrium has necessarily an electrical charge or poten- 
tial, greatest at the maximum swelling, and such charges seem 
an essential of the colloid state.24* The surface is positive or 
negative according to whether the diffusible anion or cation is 
retained in the colloid. Thus gelatin and hide fiber are negative 
in alkaline and positive in acid solutions, and it will be shown 
later that this has an important bearing on the theory of leather 
manufacture. 

“Wilson *> has extended these facts to a general theory of col- 
loids and adsorption, showing that all surfaces must possess a 
potential due to unbalanced chemical forces on the surface; and 
therefore in a liquid containing electrolytes, must condense ions 
or particles of the one sign on its surface, and repel those of the 
opposite sign; and also showing that surfaces must therefore be 
surrounded with a film of liquid of different concentration to the 
bulk, to which the same considerations and equations are ap- 
plicable as to the adsorbed solution of colloid jellies.” 

Wilson and Kern * report that “gelatin, like collagen, shows 
two points of minimum swelling with change of hydrogen-ion 
concentration, one at P,,4-7 and the other at 7.7.” They suggest 
“that the two points of minimum represent the iso-electric points 
of the gel and sol forms of gelatin, respectively.” Since Wilson 
and Kern describe the gelatin they used simply as “high-grade 
gelatin” without mentioning anything about the percentage or 
composition of the ash, their experiments must be repeated with 
gelatin of known purity before their results or deductions can 
be accepted. Their double minimum may be due to the presence 
of some adsorbed impurity such as alumina; for alum is very 
commonly used in preparing the stock or in clarifying the 


33 Procter here refers to leather manufacture. J. A. 

34 Z, Hlektrochem, 17, 579 (1911). 

84a See note 29 above. J. A. 

3% J, A. Wilson, J. Am. Chem. Soc. 88, 1982 (1916). 

sa J, A. Wilson and EH. J. Kern, J. Am. Chem. Soc. 44, 2633 (1922). 


96 GLUE AND GELATIN 


liquors, and as 8. E. Sheppard has shown ** the addition of 
alumina to an ash-free gelatin superimposed upon a maximum 
of elasticity at about p,, 8 to 9, a second maximum at about 
pa 

Jacques Loeb ** has also discussed the Donnan equilibrium 
in its relation to membrane potentials and osmotic pressure, and 
found that Procter’s formula is the correct expression for the 
Donnan membrane equilibrium, which he thinks determines 


swelling and viscosity as well as osmotic pressure and electric 
charge.*” 


Thermal Expansion of Gelatin. 


Alan Taffel** has shown that gelatin gels expand regularly 
with increasing temperature. The expansion curves resemble 
that of water, but are flatter in proportion to the concentration 
of the gel, but show no sudden inflection as does that of glass 
below its softening point. The expansion coefficients, as well 
as the specific volumes for any one temperature, are linear func- 
tions of the concentration of the gel. Variation in H-ion con- 
centration does not affect the expansion coefficient. 

Irrespective of dilution, one gram of gelatin always exhibits 
the same contraction at any one temperature. This contraction 
is 0.073 cc. per gram of gelatin at 15°, and 0.065 cc. at 32°; 
which indicates that only a fraction of the gel water contracts, 
the weight percentage being the same for gels up to 25 per cent. 
Gel contraction is not due to filling up of pores in solid gelatin 
by water. The curve expressing the relation between concen- 
tration and the calculated distance between particles, is an 
hyperbola, whereas the concentration setting-point curve ob- 
served by Sheppard and Sweet *® shows a double flexure, 
the rapid rise at 70 per cent. concentration being attributed to 
the fact that their very large molecular forces begin to come 
into play. 

Gelatin lowers the temperature of maximum density of water 


35b §, E. Sheppard, S. S. Sweet, and Anber J. Benedict, J. Am. Chem. Soc. 44, 
1857 (1922). 

36 J, Gen. Physiol. 3, 667 and 691 (1921). 

37 Loeb’s views are fully set forth in his book, ‘Proteins and the Theory of 
Colloidal Behavior,’’ New York, 1922. 

38 J, Am. Chem. Soc. 121, 1971-84 (1922). 

39 J, Ind. Eng. Chem. 13, 413 (1921). 


THE SWELLING OF GELATIN 97 


by an amount directly proportional to its concentration ex- 
pressed in grams of gelatin per 100 grams of water. This lower- 
ing is shown to be due to the ordinary volume changes of dry 
gelatin with changing temperature, and the variations in con- 
traction on imbibition of gels at various temperatures. 


Chapter 7. 
The Viscosity of Glue and Gelatin Solutions. 


The importance of viscosity measurements as a means of fol- 
lowing changes occurring in colloidal solutions, has long been 
recognized. Thus Thomas Graham in his classic paper entitled 
“On the Properties of Silicic Acid and Other Analogous Col- 
loidal Substances”! says: 

“The ultimate pectization of silicic acid is preceded by a 
gradual thickening in the liquid itself. ‘The flow of liquid col- 
loids through a capillary tube is always slow compared with 
the flow of crystalloid solutions, so that a liquid-transpiration- 
tube may be employed as a colloidoscope. With a colloidal 
liquid alterable in viscosity, such as silicic acid, the increased 
resistance to passage through the colloidoscope is obvious from 
day to day. Just before gelatinizing, silicic acid flows like an 
Ciles 

The instruments used for measuring viscosity must depend 
upon the degree of accuracy desired and the time and quantity 
of the solution available. For most scientific investigations the 
Ostwald viscosimeter ? or that of Couette,® * have been used. The 
hour glass viscosimeter of H. A. Determan ® is very useful where 
only small quantities of fluid are available. The well-known 
Engler viscosimeter is also used, and any simple graduated pipette 
will serve.® All these depend upon the time required for the 
fluid to flow through a tube. 

The MacMichael viscosimeter? operates on the principle of 
measuring by the angular torque of a standardized wire, the 
force required to cause two surfaces, one cm. apart, to move 


1Proc. Roy. Soc. London, June 16, 1864; also Pogg. Ann. 1238, 529 (1864). 

2See Ostwald-Luther-Drucker, ‘“Handbuch fiir physik. chem. Messungen,” Vol. 
3, p. 230, Leipzig, 1910. ; 

% 4H. Couette, Ann. de Chim., Ser. 6, 8, 685. A modified form is described by 
E. Hatschek, Kolloid Z. 12, 2838 (1913). 

°>H. Bechhold, ‘‘Colloids in Biology and Medicine,” p. 113. 

6 See J. Alexander, J. Soc. Chem. Ind. 25, 158 (1906). 

7™J. Ind. Eng. Chem, 7, 961 (1915). 


98 


VISCOSITY OF GLUE AND GELATIN SOLUTIONS 99 


past each other at the rate of one em. per second, at the same 
time overcoming the internal friction of the liquid under test, 
against itself throughout the intervening space. Part of the 
‘liquid moves with each surface, and the intervening layers shear 
past each other.’ 

It would unduly extend the limits of this book to enter into 
a general theoretical and mathematical discussion of viscosity, 
for as Hatschek observes,® in the present state of theory, all that 
can be deduced from viscosity measurements is that some change 
has taken place, the nature of which is either a matter for specu- 
lation or for empirical interpretation. 

Davis and Oakes ® report that “the viscosities of gelatin solu- 
tions of various concentrations at 40° conform to Arrhenius’ 
viscosity formula. 

A note of warning must be sounded, however, against the 
automatic acceptance of any formula expressing viscosity, with- 
out considering all the factors influencing the case in question. 
Mathematics is essential in solving problems, but we should 
remember that it is only a tool to work with. Granted certain 
postulates, it proceeds infallibly to direct or collateral conclu- 
sions. The danger in applying mathematics to chemical and 
physical problems is that, blinded by its logical perfection, we 
may accept erroneous postulates or neglect influential factors. 
Frequently, in Nature, unsuspected factors are discovered which 
compel us to revise our previous conclusions—for example, the 
recognition of the vitamines has rendered necessary a careful 
reconsideration of former experiments upon nutrition and a revi- 
sion of the conclusions based thereon.?° | 

Prof. Eugene C. Bingham *® points out the fact that viscosity 
as commonly reported, consists of several factors: 


8 The manufacturers of the instrument, Eimer and Amend, New York, issue 
a descriptive circular in which an accuracy of within 5 per cent. is claimed, 
which suffices for commercial work. 

®Hirst Report of the British Assoc. for the Ady. of Science, on ‘Colloid 
Chemistry and its Industrial Applications,’ 1917, p. 2. <A _ bibliography is 

appended. : 
"9a J, Am. Chem. Soc. 44, 464 (1922). 

10 For a critical discussion of the viscosity formule of A. Hinstein, Ann. der 
Phys. 19, 289 (1906), and E. Hatschek, Kelloid. Z. 7, 301 (1910), see M. von 
Smoluchowski, Kolloid. Z. 18, 190 (1916), who points out some of the assump- 
tions, omissions, or errors which limit the application of these formule. 

loa Private communication. For full details, see his book, ‘Fluidity and 
Plasticity,” p. 215, et seq. 


100 GLUE AND GELATIN 


“In measuring the deformation of any viscous material we 
assume that the deformation o of one plane is directly propor- 
tional to the shearing stress / hence v= qFr where @ is the 
fluidity and r is the distance from another plane supposed to be 
at rest. If r is unity the fluidity is evidently measured by the 
= =q. But there are very numerous substances which 
do not follow this simple law. If the deformation of the material 
is measured at different shearing stresses, it is found that the 
points fall on a curve which does not pass through the origin. 

“The intercept is known as the yield value f, and it may be 
roughly defined as the shearing stress required to cause con- 
tinuous deformation while the slope of the curve defines the 
mobility u according to the equation o—wu(F—f)r. Since 
the fluidity as ordinarily measured is dependent on the magnitude 
of the shearing stress, the fluidity is merely an apparent fluidity 
and conclusions drawn from such apparent fluidities may be quite 
illusory. On the other hand the yield value and mobility are 
two properties which are independent of the shearing stress or 
the dimensions of the instrument used. Since these properties 
also vary over an extremely wide range, they are particularly 
well suited for the identification of colloids. 

“In gelatin it appears that as the temperature is raised, the 
yield value decreases in a linear manner and becomes zero at a 
definite temperature indicating that the colloid passed into a 
true liquid. As the concentration increases the yield value in- 
creases very rapidly and the mobility decreases.” 

The striking difference between solutions of crystalloids like 
salt and colloids like gelatin, is that whereas wide variations 
in the concentration of salt in solution affect the viscosity but 
slightly, the viscosity of gelatin solutions rises sharply with 
increased concentration of gelatin. The effect of temperature 
changes is still more striking, for with, say, a 10 per cent. solu- 
tion of gelatin the difference of a few degrees will change a fluid 
into a solid." 

Viscosity 1s consequent upon the internal friction of the sub- ° 
stance in question; in fact, this latter expression is frequently 


slope 


1 For a general discussion of the viscosity of colloids, including gelatin, see 
e.g. Wo. Ostwald, “Handbook of Colloid Chemistry’; T. B. Robertson, ‘“‘The 
Physical Chemistry of the Proteins,’ Ch. 13; and W. D. Bancroft, ‘Applied 
Colloid Chemistry,’ p. 190. 


VISCOSITY OF GLUE AND GELATIN SOLUTIONS 101 


used as being synonymous with viscosity. It is immediately evi- 
dent, therefore, that viscosity is intimately dependent upon the © 
number, size, electric charge, and degree of hydration and aggre- 
gation of the particles of the dispersed substance. Many (e.g. 
Ostwald and T. B. Robertson) incline to the belief that protein 
solutions have a net-like structure which is responsible for the 
high viscosity they exhibit. 

This view is in accord with recent experiments of the British 
Adhesives Research Committee,** who tested the effect of vari- 
ous salts on the jelly strength, surface tension, tensile strength 
and viscosity of glues, and found that a salt which produces 
great dispersion will tend to form a glue of lower viscosity than 
one of small dispersive capacity. Their acceptance of the state- 
ment that gelatin itself is not a good adhesive, is, of course, 
unwarranted, 

One factor not generally stressed is the kinetic factor. With 
increasing aggregation, the Brownian motion of dispersed parti- 
cles diminishes rapidly, so that although there may be no actual 
permanent structure in a gelatin solution, there are at any instant 
a certain number of slowly moving or practically motionless 
groups, whose number and size increase as gelatinization is ap- 
proached. These large groups greatly increase the internal fric- 
tion or viscosity, and anything which fosters their formation 
will have a like result. With crystalloids the rapid kinetic 
motion of the dispersed particles tends to prevent them from 
exercising a material effect on viscosity. 

It is interesting to note that there seems to be a zone of 
maximum colloidality or viscosity,‘* which corresponds roughly 
with the so-called colloidal zone of dispersion (about 5 uu to 
100 wu). Thus the experiments of M. H. Fischer ** show that 
with increasing molecular size soaps make more viscous solu- 
tions; while on the other hand when cream is homogenized, the 
decrease in the size of the fat globules is also accompanied by 
higher viscosity. The experimental facts reviewed by Wo. 
Ostwald 14 indicate the existence of such a zone in the case of 
gelatin, on either side of which its viscosity diminishes. 


11a First Report, p. 24, London, 1922. 

122See J, Alexander, J. Am. Chem. Soc. 43, 434 (1921). 

13“Soaps and Proteins”; Chem. Eng. 27, 155 (1919). 

14 “‘FfTandbook of Colloid Chemistry,’ 2d ed., p. 158 et seq. See also Bogue, 
“Gelatin and Glue,” p. 191 et seq., and p. 217. 


un 


102 GLUE AND GELATIN 


R. H. Bogue * has investigated the relation between the vis- 
cosity and concentration of gelatin sols, and finds no constant 
ratio between the two, but finds that variations in the hydrogen 
ion concentration cause wide variations in viscosity and in the 
volume occupied by a unit weight of gelatin. Isoelectric gelatin 
(H-ion concentration = 2 < 10°) had the lowest viscosity and 
the lowest degree of solvation; gelatin chloride (H-ion concentra- 
tion = 3.1 X 10*) had the highest, and calcium gelatinate (H-ion 
concentration = 2.5 * 10°) is intermediate. Solvation and vis- 
cosity appear to be parallel functions, according to Bogue. 

Whether the purely ‘‘chemical” or the physical explanation 
be assumed, these facts indicate the potent influence on viscosity, 
of changes in the size of the particles constituting the dispersed 
gelatin. Bogue assumes an equilibrium: surface tension (of 
dispersion medium) = solvation potential (of dispersed phase), 
with a possible reversal of phases at higher concentrations. The 
degree of aggregation of the primary particles seems to be an 
important factor, there being a balance between the attraction 
of the solvent for the particles and the attraction of the particles 
for each other which makes the secondary groups more or less 
hydrous according to circumstances. 

Bogue 1** after a critical review of several theories of gel 
structure, including Loeb’s occlusion theory,” concludes that 
many contemporary investigations have been found to support a 
eatenary or fibrillar hypothesis which he epitomizes as follows: 
“The sol consists of slightly hydrated or swollen molecules 
united into short chains. When the temperature falls the threads 
increase in length and number, and their power of water absorp- 
tion increases, resulting in an increase in viscosity. A solid jelly 
results when the relative volume occupied by the swollen molecu- 
lar threads has become so great that freedom of motion is lost, 
and the adjacent heavily swollen aggregates cohere. The 
rigidity is dependent upon the relative amount of free solvent 
in the interstices of the aggregates, and on the amount of solvent 
that has been taken up by the gelatin in a hydrated or imbibed 
condition. The resiliency or elasticity is dependent upon the 
length and number of the catenary threads. Solution is the 

18 J, Am. Chem. Soc. 43, 1764 (1921). 


18a R. H. Bogue, J. Am. Chem. Soc. 44, 1843 (1922). 
1b J, Loeb, J. Gen. Physiol. 3, 827 (1921); 4, 78, 97, 351 (1921-22). 


VISCOSITY OF GLUE AND GELATIN SOLUTIONS 103 


reverse of gelation. Swelling is determined by osmotic forces 
and the Donnan equilibrium.” 

By the additions of increasing amounts of alcohol, the micel- 
lular groups are progressively dehydrated and shrunken, the 
gelatin solution becomes opalescent and its viscosity drops. 

L. Arisz 7° dissolved 10 per: cent. of gelatin in glycerin of sp. 
ger. 1.176 (containing 32 per cent. of water), and noted the 
changes in viscosity at different temperatures and over different 
periods of time. At 65° the viscosity was unchanged. after 24 
hours, but at higher temperatures it suffers a gradual decrease 
’ which amounts to 8 per cent. after 30 minutes at 95°. If the 
viscosity of water be considered as unity, the viscosities of the 
10 per cent. gelatin-glycerin solutions determined with an 
Ostwald viscosimeter after equilibrium had been reached at 
lower temperatures were: 


Temperature Viscosity 
eine co ceccecsvcce 222 
a rr 415 
eM cc om ye hese Salvi ne about 950 
BIO ok wt ct asec ences less than 4,200 
PE cic ids sia a oldie We idee ws about 5,000 
Toco chia a piy) scave « a.n sin ao Os over 30,000 


That is, at 44° the solution had practically gelatinized. Very 
low temperatures inhibit gelatinization of dilute gelatin solu- 
tions. Thus Arisz found, contrary to expectation, that 114 per 
cent. solution which set after 3 days at 20°, was still fluid after 
2 weeks at 2°. This is probably because of the reduced kinetic 
motion of the gelatin particles consequent upon low temperature. 
He also found that the viscosity as well as the Tyndall phe- 
nomenon of gelatin solutions depend upon their previous thermal 
history. The establishment of the viscosity equilibrium takes 
time, and there is a time lag observable upon warming or on 
cooling the solutions. (For full details of Arisz’s elaborate ex- 
periments, the original must be consulted.) 

The viscosity of a gelatin solution is influenced by time; tem- 
perature; concentration; mechanical agitation; innoculation or 
seeding with a more aggregated solution; hydrolysis, enzymic 
or bacterial decomposition; the addition of acids, alkalis, salts, 
-and of a wide variety of non-electrolytes. 


16 Kolloidchem. Beihefte 7, 1 (1915). 


104 GLUE AND GELATIN 


The time and temperature effects are familiar to everyone; 
it is a matter of common knowledge that standing and chilling 
increase the thickness of solutions of glue and gelatins. These 
effects may be seen quantitatively by tabulating some of the 
results of P. von Schroeder ** and 8. J. Levites.'’ 


INCREASE IN Viscosrry or GELATIN SoLUTIONS wiTH TIME 
Pe Levites (det. with Ostwald 





P. von Schroeder viscosimeter ) 
Afterelapse At At At After elapse At At 

of 21.0° 24.0° 81.0° of a5 26° 
ANI a, oo sl eo 1.65 1.41 ase eee 2.94 
One: gine aa A 1.69 1.41 15 min. .5.98 eee 3.00 
Lb ee ye siete eo 1.74 1.42 30.“ os ieee 3.05 
BUM iomect ed. 1a 1.80 1.42 45 SO eee 3.13 
G0 ae ASG 1.90 1.42 60 “goes 3.19 
vA Tot osha atte 3.24 
90 § "eee 3.29 

11 hours.... gelatin- gelat.in 

ized 24 hrs. 


R. H. Bogue (loc. cit., infra.) also gives a table showing the 
variation in viscosity of 6 hide glues and 7 bone glues at tem- 
perature between 150° and 83° F. (67° and 28° C.). 

The well known effects of increase in viscosity due to increas- 
ing concentration of gelatin is shown in the following tables 
taken from 8. J. Levites (loc. cit.) and R. H. Bogue: ” 


Hydrolyzed Gelatin (8 Glutin) non- 


Gelatin (a Glutin) gelatinizing 
Cone. 
Conc. of Gelatin Viscosity of B Glutin Viscosity 
in per cent. at 35° in per cent. at 35° 
O25 5 hota re 1.10 0.50 . 0.4 Vee 1.186 
O.O0s tee a 122 1,00... 6.35 eee 1.362 
O75 eee eles ae 1.32 15). 2 oo) oe 1.382 (7) 
1 OO er cee horn eee 1.46 2.00 +. 3cs <0 See 1.432 
1 D0 eet se cee 1.75 3.00. «scx Ae eee 1.603 
DUO se eee 2.05 4:00... eee 1.856 
BEY Ue peak od ere 2.96 
EFFECT OF CONCENTRATION ON THE VISCOSITY OF GLUE (BocuUE) 
Ratio of Glue Hide Glues Bone Glues 
to Water Hy, H, H; Hs Bi B, B, By 
10:t0:17Gt ee 42.0 42.4 426 418 40.8 418 41.0 41.4 
20.401 D0U. occ 46.4 46.4 44.2 42.6 43.0 440 42.4 414 
30 to 160....... 59.8 58.4 49.4 47.0 48.4 50.6 446 416 
40 to 140....... 99.8 83.6 60.0 56.4 59.8 61.4 50.0 43.4 


7 Z, Physik. Chem. 45, T5 (1903). 
18 Kolloid Z. 2, 210 (1907). 
19 Chem. Met. Eng. 23, 61 et seq. (1920). 


VISCOSITY OF GLUE AND GELATIN SOLUTIONS 105 


Mechanical agitation decreases the viscosity of gelatin solu- 
tions, just as it does with many other colloids. Consequently 
in making or using glue or gelatin unnecessary agitation is to be 
avoided; for although with slight stirring the dispersing effect — 
is reversible or negligible, where violent agitation is used there 
may be produced an irreversible disintegration. 

The effect of an addition of aged gelatin solution in accelerat- 
ing the gelatinization of gelatin solutions has been observed by 
H. Garrett.2° Wo. Ostwald regards this as due to a chemical 
change in the gelatin, probably to its hydrolytic cleavage; but 
the more likely explanation is that the aged gelatin furnishes 
“nuclei” upon which larger aggregates are formed, thus speeding 
up the aggregation which finally results in gelatinization. 

Any agency which produces hydrolysis or degeneration of 
gelatin, will reduce the viscosity of the solutions. Enzymes 
(pepsin, trypsin) and bacteria bring about the result, and bac- 
terial decomposition must be carefully guarded against by anti- 
septics (chloroform, toluol, phenol), refrigeration, or steriliza- 
tion or it will vitiate the results of many experiments. 

The previous thermal history of gelatin, however, is a most 
important factor in determining its viscosity, as is evident from 
the following table from P. von Schroeder: 


Errect of HEATING ON VISCOSITY OF GELATIN SOLUTIONS 


Hours heated to Viscosity of 
about 100° . 1 percent.solu. 2 percent.solu. 3 percent. solu. 

SL ye 1.29 1.75 — 
A 123 1.55 — 
Beye pha.b gc asc-0 1.20 1.49 = 
lk Liz 1.47 1.76 
ee 1.15 — 
re Eas ois, 5 isan 2 1.14 1.37 1.68 
“OL ee 1.13 — = 
2 Gn i — — 
01) A re — 1.30 1.54 
OO oS _— 1.25 1.47 

MUP EN ce sind s vines « — 1.24 1.42 

ess ee _ 1.23 1.40 

(OS hs eee — 1.22 1.39 

NAL SS ee eee — 1.22 1.39 


‘Practically, this means that continued heating rapidly reduces 
the viscosity of glue and gelatin solutions, or makes them “run 
thin.” The longer a glue solution is heated the lower becomes 
its adhesive value. 

20 Diss. Heidelberg, 1903; Phil. Mag. (6), 6, 374 (1903). 


106 GLUE AND GELATIN 


Influence of Added Substances on Viscosity. 


The influence of added substances on the viscosity of gelatin 
varies widely, depending on the kind and quantity of the sub- 
stance added, and also the time. In summarizing the results 
of P. von Schroeder,” 8. J. Levites 7? and Gokun,?* Wo. Ostwald 4 
states that with gelatin the initial value of the viscosity follow- 
ing the addition of a salt follows the general rule of mixtures: 
salts which raise the internal friction of water affect the gelatin 
similarly and vice versa. A very different final value is ap- 
proached asymptotically. 

The following is a tabular result of some of von Schroeder’s 
work: 


IncrEasp (-+-) or Decrease (—) IN Viscosity or 1 Per Cent. GELATIN 
SoLtuTIon 1 Hour Arter THE ADDITION oF SALTS 


Na K NH, Mg° li 
SOs 710M: 4s = dat 0.22 + 0.17 
SOF 1/8n 2 8 + 0.33 + 0.09 +017 0.44 -+ 0.24 
BOJ— 1/4 esck., +101 —— + 0.48 + 0.94 0.47 
SOeee Lio nes + 7.63 ee + 1.64 ie <r 
Cle /R ie ee oa Seay =~ O15 + 0.10 + 0.05 
Cea nese =~ 0),12 — 0.08 OSL + 0.20 == 0,02 
Uri? tie a to + 0.01 — 0.03 =n) + 0.32 +. '0.20 
cart Niet <a —+ 0.000 t= 0.20 = 0:20 = ae 
Ne 1/Sn ee ee — 0.00 — 0.04 a — 
NOg 1/4 ae 002 — 0.15 <= 0.56 ca — 
NO; —1/2n .2.., —011 4 024550 _ es 
Nae Tn nated: — 020%. a0, 32m een ee so 7 


« 


H. Bechhold and J. Zeigler *° report the following results on 
melting points: 


Melting 

Point °C. 
10 per cent. gelatin + 2 mol. Na:SO, ....2.. 5 34.2 
10 Mr slap bie ee nul eurbg Ora ey eee 31.6 
Neen “ o-I1 mol. NaCl .....22. 2a 28.5 
LORE: = ‘ -+F 1 mol. Na: To 2... ce 10.0 


With salts in rather high concentrations, the effect on solidifi- 
cation time, and on jelly strength or melting point, runs as 
follows: for amons, sulphate > citrate >tartrate > acetate >chlo- 
ride>nitrate > bromide > iodide > ae > benzoate > 


21Z, physik. Chem. 45, 75 (1908). 

2 Kolloid Z. 2, 210 (1907). 

23 Kolloid Z. 3, 84 (1908). 

24 “Fandbook of Colloid Chemistry,” 2d ed., p. 169. 
25 “Colloids in Biology and Medicine,” p. 162. 


VISCOSITY OF GLUE AND GELATIN SOLUTIONS 107 


salicylate. According to Bechhold the action of cations is of 
smaller importance. 

The results of R. H. Bogue 2¢ on glues are summarized by him 
as follows: 

Practically all the substances added lowered the gel strength. 

Strong (9N) sodium hydrate had the greatest effect, followed 
by potassium iodide, strong (9N) sulphuric acid, sodium sul- 
phate, acetic acid and magnesium chloride. The effect of the 
others was small. 


The viscosity was raised constantly by magnesium chloride,. ° 


chloral hydrate, and sodium silicate. 

The viscosity was raised to a maximum, after which it fell 
more or less rapidly, by sodium hydrate, disodium phosphate, 
and acetic acid. | 2 zs 

The viscosity was lowered constantly by potassium iodide, 
sulphuric atid, phosphoric acid, and sodium sulphate. 

There was no appreciable effect on the viscosity due to sodium 
chloride and magnesium sulphate. 

Monosodium phosphate produced a sharp drop of one second 
at 0.1 per cent., followed by a sharp rise of 31% seconds at 0.5 
per cent., after which it rose a little further and then dropped 
again. 

The disparities between von Schroeder’s results on gelatin 
and Bogue’s results on glue indicate perhaps a difference in ex- 
perimental procedure (temperature and time of heating before 
taking viscosity, etc.), or else a material difference in the be- 
havior of glue and gelatin, or perhaps a difference in ‘‘impurities”’ 
or hydrogen ion concentration. As Ostwald remarks in a foot- 
note, pure gelatin would, perhaps, show totally different results 
from those of von Schroeder. In any event it is obvious that 


the experimental facts need to be carefully redetermined, having © 


in mind all the variable factors which recent investigations have 
shown materially effect the properties of gelatin.. Thus the 
experiments of D. Jordan Lloyd ?* and J. Loeb ** were performed 
with gelatin containing about 0.1 per cent. of ash. Loeb’s fig- 
ures (loc. ¢it., p. 35) indicate that one sample of his gelatin 


contained mainly calcium and iron phosphates. As C. R. Smith 2° 
2 Chem. Met. Eng. 23, 61 et seq. (1920). 
27 Biochem. J. 14, 584 (1920). 
28“‘*Proteins and the Theory of Colloidal Behavior,’ New York, 1922. 
229 J. Am. Chem. Soc. 48, 1850 (1921). 


108 GLUE AND GELATIN 


has shown how to prepare absolutely ash-free gelatin, much of 
the preceding work must be repeated. 

The view of H. R. Procter, J. A. Wilson and J. Loeb *° is that 
gelatin forms definite hydrolyzable salts, e.g. gelatin chloride 
with HCl and sodium gelatinate with NaOH. Loeb believes 
that the effect of acids, alkalis, and salts on the viscosity, swell- 
ing, osmotic pressure, and general behavior of gelatin is explain- 
able on the basis of the Donnan theory of membrane equilibria. 
Loeb (loc. cit., p. 204) concludes from his experiments that “it 
~ seems that the viscosity of the solutions of proteins is primarily 
a function of the relative volume occupied by the protein in 
solution,” but that “the difference in the viscosity of solutions 
of gelatin and crystalline egg albumen cannot be ascribed to 
differences in the degrees of hydration of the individual protein 
ions since at the isoelectric point the protein is not lonized.” 
(Loeb’s measurements were made at or near this point.) 

Loeb believes that gelatin solutions contain submicroscopic 
particles of solid jelly, and that a Donnan equilibrium arises be- 
tween these and the surrounding solution. This equilibrium 
regulates the amount of water occluded by the submicroscopic 
particles of solid jelly floating in the gelatin solution, and the 
high viscosity of gelatin solutions is due to the presence of these 
swollen particles which increase the relative volume occupied 
by the gelatin in solution. 

Loeb here criticizes the theory of Wo. Pauli, who holds * 
that the viscosity of protein solutions depends primarily upon 
hydrated protein 1ons. Wo. Ostwald and M. H. Fischer likewise 
disagree with Pauli, but uphold the aggregation hypothesis 
which is condemned by Loeb. Strange to say, Ostwald, Loeb, 
and Fischer draw about the same picture of what happens in 
gelatin, although they differ as to the mechanism by which the 
result is brought about. | 

“The quantities of water which can be occluded in a Gate 
jelly of gelatin are enormous. If we assume the molecular 


30 The views of H. R. Procter and J. A. Wilson are set forth in many journal 
articles, a good abstract of them being found in the “First Report on Colloid 
Chemistry and its Industrial Applications,” London, 1917, pp. 5 et seq. (by 
Procter), and the Third Report, London, 1920, pp. 48 et seq. The various 
journal articles of J. Loeb are collected in his book, ‘Proteins and the Theory 
of Colloidal Behavior,’”’ New York, 1922, in which his views and many experi- 
ments which he believes confirm them, are set forth at length. 

31 ‘*Kolloidchemie der Hiweisskérper,’ Dresden and Liepzig, 1920. 


VISCOSITY OF GLUE AND GELATIN SOLUTIONS 109 


weight of gelatin to be of the order of magnitude of about 
12,000, a solid gel of 1 per cent. originally isoelectric gelatin 
contains over 60,000 molecules of water to 1 molecule of gelatin. 
It is out of the question that such masses of water could be held 
by the secondary valency forces of the gelatin and water mole- 
cules. . . . All the experiments described agree with the occlu- 
sion theory but not with the hydration theory.” ** 

Occlusion is believed by most physicists and chemists to be 
due to residual stray fields of force at the surfaces involved. H. 
Freundlich ** would probably call it capillarity, while some 
would call it absorption, adsorption, or sorption, the latter term 
being suggested by McBain ** as being free from theoretical 
assumption as to its cause. 

R. 8. Lillie ** observed that neutral salts depress the osmotic 
pressure of gelatin solutions, a result explained as due to aggre- 
gation consequent upon the precipitating action of the salts. 
But salts decrease the viscosity of gelatin solutions, and as Loeb 
properly concluded from some of his experiments that aggrega- 
tion increases the viscosity of gelatin ** he argues that the salts 
cannot cause aggregation. 

Loeb here entirely overlooks the zone of maximum degree of 
colloidality referred to on p. 101. Aggregation increases viscosity 
only up to a certain point or zone, after which further aggrega- 
tion may reduce viscosity. Below this zone the kinetic motion 
of the particles seems to be a controlling factor, while above it 
diminution in the free surface of the particles or in the amount 
of water they hold, tend to reduce viscosity. Thus karaya gum 
powder whose water-imbibing capacity has been diminished by 
heating, yields less viscous solutions than the original gum. The 
heated gum particles show inferior swelling because their con- 
stituent submicroscopic molecular groups remain more aggre- 
gated (that is less dispersed) ; more water remains “free” (unad- 
‘sorbed or unoccluded) and the viscosity is therefore less than is 
the case with the unheated gum. A similar condition exists 
with minerals like clay, and emulsions like cream; their viscosity 
increases with subdivision of the dispersed phase. 


82 Loeb, loc. cit., pp. 229-230. 

33 “*Kapillarchemie,’’ Leipzig, 1909. 

34 J. W. McBain, Phil. Mag. (6), 18, 916 (1909). 

3% Am. J. Physiol. 20, 127 (1907). 

36 J, Gen. Physiol. 4, 97 (1921-2) ; also loc. cit., pp. 16 and 114. 


110 GLUE AND GELATIN 


The importance of the hydration (hydration, swelling, water 
occlusion) of secondary groups or micellular aggregates in in- 
creasing viscosity, is obvious, and depends upon the total free 
surface of these groups. Thus the atoms or molecular groups of 
metals draw together so powerfully, that the water films about 
their nascent colloidal dispersions are squeezed out; the dispersed 
phase is dehydrated and the viscosity very low, most of the 
water being in the dispersing phase. When the dispersed phase 
is highly hydrated as with gelatin, gum karaya, etc., the viscosity 
is high, most of the water being in the dispersed phase. With 
emulsions, increasing subdivision of the dispersed phase brings 
increase in active surface and in viscosity, although presumably 
the individual particles of dispersed oil are hydrated only at 
their exterior. Comparing these three types, the fact that the 
gelatin particles have interior surfaces and are at least duplex, 
becomes evident. 

With cooling gelatin, however, the zone of maximum colloid- 
ality or viscosity is approached from the opposite side, and we 
have an increase in viscosity due to aggregation of the dispersed 
phase. Sodium salts of the fatty acids also illustrate this ap- 
proach, as their solutions become more viscous and hold more 
water with increase in the molecular complexity of the fatty 
acid.27 From Bechhold’s table (see Chapter 3, p. 50) it may be 
seen thatthe particles in gelatin solution are of the order of 4 up, 
so that there is considerable latitude for increase in viscosity due 
to aggregation, before passing beyond the colloidal zone. Inter- 
esting results should be obtained by taking the viscosity of 
gelatin heated to 110° for various periods of time, for the results 
of Bogue *§ indicate that a rise followed by a fall in viscosity 
may be expected. 

M. A. Rakusin *® in a monograph entitled “The Animal Skin 
as an Amphoteric and Colloidal Protein” quotes the analytical 
results of von Schroeder and Passler *° showing that animal skins 
of diverse origin show a remarkable uniformity in ultimate 
analysis. 

‘7 For further discussion of this view see J. Alexander, ‘‘The Zone of Maximum 
Colloidality. Its Relation to Viscosity in Hydrophile Colloids, Especially 
Karaya Gum and Gelatin.” J. Am. Chem. Soc. 43, 484 (1921). 

88 Chem. Met. Eng. 23, 61 et seq. (1920). 


8° Kolloidchemische Beihefte 15, 103-184 (1922). 
40 Dingl. polytech. J. 287, Heft 11, 12 and 13 (1893). 


VISCOSITY OF GLUE AND GELATIN SOLUTIONS 111 


Skin b. H N 
OSB CRVETOZC)) ce. ecko ees 50.47 6.46 17.76 
OT. A ee 50.21 6.46 17.78 
US edie Aa Se 50.02 6.43 17.67 
ia. a ee He ae a 50.20 6.44 17.93 
Od 49.90 6.31 17.84 
Rhinoceros (average) ........ 50.19 6.37 18.04 
OS Eire. ea ede «ars fuse ere 50.31 6.35 17.48 
DOSS i ra 50.34 6.38 17.42 
SON EO. gpk ble Si ree 50.19 6.49 17.05 
PP NIIS Er cle seals acc tlee e's 50.14 6.37 17.38 
LUGE ee a ne an 50.26 6.45 16.97 
Ch. onl ieee 51.10 6.51 17.05 
ON a a 49.91 6.35 17.72 


The figures apply to water-free substance. In the case of the 
ox and rhinoceros the average refers to pieces of hide from dif- 
ferent portions of the animal. The gelatin was one sold by 
Gribler for bacteriological purposes. 

From these figures von Schroeder and Passler concluded that 
hide substance and gelatin represent a chemical individual, a 
conclusion in which Rakusin unwisely concurs, for in the light 
of our present knowledge of isomerism, stereoisomerism, poly- 
merism, tautomerism, etc., its danger would be obvious even in 
the case of substances far less complex than the proteins. With 
ossein and gelatin where we have an as yet undefined complex 
of polypeptides and amino-acids, the impossibility of such a 
conclusion becomes immediately manifest. 

Here again we have a striking instance of the desirability of 
refraining from rushing to frame a plausible theory which will 
fit a certain set of facts ‘within the limit of experimental error,” 
without considering other known experimental facts and the 
possibility of unsuspected but potent factors. In the case of 
the proteins colloidal protection is probably such a factor, and 
as Bismarck once said the things he most feared were the ‘‘im- 
ponderables.”’ 

Rakusin states (loc. cit., p. 110) that hide powder, in con- 
tradistinction to gelatin, contains a small quantity of sulphur 
that can be split off, for with lead or bismuth salts in the pres- 
ence of alkali it gives a slight precipitate. Md6rner *! attributed 
this sulphur to the presence of cystine, but Rakusin claims to 
have proven that “the sulphur in gelatin is fixed as chondroittin- 


“1 Oppenheimer, Handb. d. Biochem. d. Tiere u. des Menchen 1, 331 and 395 
(Jena 1909).- i 


112 GLUE AND GELATIN 


sulphuric acid which exhibits no protein reaction whatever, but 
reacts with barium chloride; its dextro-rotation distinguishes it 
from sulphuric acid.” From this it is evident that Rakusin 
used a gelatin containing chondrin or some similar impurity, and 
it leads one to suspect that the sulphur in hide powder may come 
from some impurity such as keratin, for example. 

Herzog and Adler # showed that both hide and gelatin exhibit 
an apparent negative adsorption—that is, they selectively adsorb 
the solvent leaving the solute more concentrated. Rakusin says 
that in dyeing hide a positive and negative adsorption occur 
simultaneously,** but this seems to be a rather recondite way 
of saying that the hide takes up both water and dye. He also 
asserts with great positiveness that the combination between 
dye and hide is chemical, but says that crystal violet, which 
washes out with alcohol, constitutes “a preliminarily inexplicable 
exception.” So too did methylene blue, which was dissolved out 
by both boiling water and by alcohol; and in this case Rakusin 
promises to repeat the experiment with the purest dye obtainable. 

Hide powder and gelatin were both dyed by methyl orange 
(dimethylanilin-azo-benzolsulphonic acid), but the adsorption 
product showed the curious anomaly of being reversible in boil- 
ing water but irreversible in 95 per cent. alcohol. 

Rakusin also discusses at length the tanning of hide by tannin, 
formaldehyde, aldoses, phenols of various kinds, and homologous 
substances, picric acid, naphthols, chinone, ‘“‘neradol,” and alum, 
iron, and chrome salts. For full details reference must be made 
to his monograph. 

Non-electrolytes in general have slight action on the viscosity 
of gelatin, although many of them materially influence its jelly 
strength and melting point. Bechhold and Zeigler report the 
following: 


M.Pliam- eG 
10 per cent. gelatin. ........:6. 00008040 4s eee 31.66 
+1 mol. grape sugar;...eeee 32.25 
1D Sc anaes “ A-2 “ glycerin |... eee 32.17 
LO gas «+2 “~ alcohol’ “<c\2 553m 30.00 
Len a “ ---1 >“ urea )..5.4.. ae 260 


Furfurol, rescorcinol, hydroquinone and pyrogallol also lower 
the apparent rcHitie point of gelatin. 


“ Kolloid. Z. 2, Suppl. II, 3 (1908). 
43 See also M. Rakusin and G. Pekarskaja, J. Russ. Chem. Ges. 1917, 1899. 


Chapter 8. 


Collagen or Ossein. 


Collagen? (literally glwe-former) is, as its name indicates, 
the parent substance of glue and gelatin. It is a substance par- 
ticularly characteristic of mature vertebrates, and is found in 
all of them with the exception of the border-line Amphioxus 
lanceolatus, according to Hoppe-Seyler. This same investigator 
reports that jelly-forming tissue is practically never met with in 
invertebrates, although he found some in two cephalopods, 
Octopus and Sepiola (the devil-fish and the cuttle-fish). 

From this it is obvious how vitally collagen is involved in 
bone formation. In fact, bone consists essentially of tricalcium 
phosphate and calcium carbonate deposited in collagen which 
acts as a colloidal protector and inhibits their crystallization. 
H. Bechhold? has discussed some of the theories of ossification, 
and describes experiments of R. E. Liesegang, who simulated 
bone formation by allowing disodium phosphate and calcium 
chloride to diffuse toward each other in gelatin jellies. Pauli 
and Sameé found that serum albumen increases the solubility of 
calcium carbonate 475 per cent., and of calcium phosphate 90 
per cent.; but with the cleavage products of albumin, the figures 
are reversed. Since human bone ash contains about 85 per cent. 
of Ca,(PO,), and 9 per cent. CaCO, Bechhold believes that the 
disposition of the bone salts is consequent upon or accompanies 
the disintegration of cells or tissues, which corresponds with the 
histological evidence. 

In pathological cases bone formation may be inhibited, as in 
rickets; or bone already formed may be destroyed, as in osteo- 
malacia. The disposition of lime is evidently closely bound up 
with variation in the protective action of the body colloids, too 
high a degree of colloidal protection working against deposition. 


1Ossein is collagen derived from bones. 
a2“Colloids in Biology and Medicine,” p. 268. 
3’ “Beitrag zur einer Kolloidchem. Theorie des Lebens,’”’ Dresden, 1909. 


113 


114 GLUE AND GELATIN 


Selective adsorption also seems to be a factor in bone formation, 
as well as in the allied phenomenon of calcification, of tubercles 
for example. The presence of a vitamine (possibly certain fatty 
acids) in foods is a factor, and sunlight also exercises an in- 
fluence. 

W. von Gaza‘ in dealing with the changes of tissue colloids 
in the healing of wounds, says that connective tissue cells have 
the specific property of forming collagen from simpler albumin- 
ous substances. This collagen is held in colloidal solution ini- 
tially because of the presence of the acid oxidation products of 
life (especially CO,). As the colloidal solution of collagen 
accumulates in the cell, oxidation diminishes and finally a re- 
versal of reaction occurs—neutrality instead of acidity. The 
formation of collagen is thus analogous to lignification, the for- 
mation of lignin from cambial sap, a phenomenon which has 
been investigated by Wislicenus.’ Collagen and lignin both func- 
tion as supporters and protectors, and are marked .by great 
stability under normal conditions of life. Even after death this 
stability persists as is evidenced by fossil bones and wood. 

After growth, collagen remains unchanged apart from col- 
loidal syneresis. In order that healing of a wound (i.e. in the 
skin) may take place, it 1s necessary that the paraplastic col- 
lagen be brought into a fluid or semi-fluid condition, for it is an 
old maxim that corpora non agunt, nisi fluida. Fluidification 
seems to be brought about by local accumulation of acid due to 
interruption of the normal circulation consequent upon the 
wound. Similarly collagen must first be swollen to a certain 
extent by hydrochloric. acid before pepsin will disintegrate it. 
It is not, however, attacked by leucocytes or by the tryptases of 
the tissues. 

R. H. A. Plimmer ® points out that pepsin attacks both gelatin 
and ossein (collagen), while trypsin attacks only gelatin. This 
is held to indicate that ossein has a “closed ring” or anhydride 


4 Kolloid Z. 23, 1 (1918). 

5 Kolloid Z. 6, 19 (1910). 

6“Chemical Constitution of the Proteins,’ 2d ed., Part II, p. 11. A. W. 
Thomas and I. L. Seymour-Jones (article in press) have been able to attack 
collagen with trypsin under certain conditions. The action is most rapid at 
Py 5.9 and is not materially accelerated by soaking. Collagen without trypsin, 
slowly hydrolyzed at 40°, as was observed in a blank determination. Fine hide 
powder was attacked by trypsin much more rapidly than coarse hide powder, 
showing the great effect of surface. 


COLLAGEN OR OSSEIN 115 


form, which only pepsin can open. A more likely explanation 
that in ossein the constituent molecular groups are more highly 
dehydrated and closer together than in gelatin, and this con- 
dition is relieved to some extent by the acidity essential to the 
activation of the pepsin. This is not inconsistent with the fact 
that, as Emil Fischer and Abderhalden have shown, the action 
of enzymes on the comparatively simpler polypeptides depends 
upon the configuration of the latter, as is shown in the case with 
sugars.‘ 

A. Ewald,® working with collagen derived from the tendon of 
the mouse, studied the shortening of its fibrils on heating in 
water, which is very marked if the collagen is first purified by 
tryptic digestion. ‘The behavior of collagen treated with formal- 
dehyde is so characteristic that F. C. Thompson thinks ® it may 
serve as a new qualitative test. At 93° C. such fibers shrink 
to one third of their original length, but regain half the loss 
upon soaking in cold water. On treating again at 69° C. they 
once more contract to one third; but their original length is 
completely regained by prolonged soaking in cold water. 

Hofmeister observed that on heating dry gelatin to about 130° 
it became insoluble, being reconverted into ossein; and he held 
that this indicates that ossein is an anhydride of gelatin. J. 
Alexander *° believes the insolubility consequent upon the re- 
moval of the protective aqueous films, the constituent molecules 
or particles of the gelatin approaching so close as to form an 
irreversible gel. An analogous condition exists with silica and 
with clay, where dehydration up to a certain point is reversible, 
after which the material will not hydrate or redisperse again 
within a reasonable time. 

Thomas and Kelly 1 report the isoelectric point of collagen 
as Py, = 9, whereas Porter 1°” reports it as 4.8. 


_ While the main sources of collagen are hide or skin and bones, 
it is found also in tendons or sinews, connective tissue, in the 
cornea and sclerotic coat of the eye, and in the scales of fishes. 


7See S. B. Schryver, ‘‘Allen’s Commercial Organic Analysis,” 4th ed., Vol. 8, 
p. 469. 

8Z, Physiol. Chem. 105, 115 (1919). 

9«S  C¢. I. Annual Report on Appl. Chem.,”’ 1919, p. 361. 

10 “‘Allen’s Commercial Organic Analysis,” 4th ed., Vol. 8, p. 586. 

10a J, Am. Chem. Soc. 44, 195 (1922). 

10 J, Soc. Leather Trades Chem. 5, 259 (1921); 6, 83 (1922). 


116 GLUE AND GELATIN 


W. 8. Ssadikow ™ has made experiments with native tendocol- 
lagen, which is a compact mass of fibrous structure. If dehy- 
drated by alcohol and carefully dried over sulphuric acid at 
room temperature and then up to 105°-180°, the fibrous struc- 
ture is not lost. The collagen does not become brittle and can- 
not be ground. Above 180° it browns and becomes brittle, the 
‘“intracolloidal” water being driven off. If dried in the presence 
of its adsorption water, tendocollagen loses its white color and 
opacity, and assumes the hyaline form, which may also be pro- 
duced by heating it for some time with hot water on a hot water 
bath, or by treatment in the cold with weak caustic alkali or 
strong alkali sulphide. 

Resoaking the hyaline tendocollagen in water restores the 
fibrous condition, unless the hydrolytic influence has exceeded a 
certain limit, when the change becomes irreversible, i.e. soaking 
one month in 2 per cent. KOH, or treatment with a saturated 
solution of Na,S. On treatment with CS,, thionyltendocollagen 
is formed, which shows characteristic reactions and is quite re- 
sistant to hydrolysis. The amount of sulphur bound depends 
upon the previous treatment of the collagen, and varies from 
0.12 to 3.05 per cent. 

Ssadikow also reported !* on the action of carbon bisulphide 
on gelatin and on ossein. To a hot solution of gelatin he added 
1 per cent. of powdered caustic soda or calcium hydroxide, fol- 
lowed by a few ce. of carbon bisulphide. The mixture was then 
allowed to set. ‘The reaction was evidenced by the development 
of a brown color, the evolution of ammonia and hydrogen sul- 
fide, and the precipitation of thiogelatin (thioglutin). On slowly 
drying thioglutin at ordinary temperatures, an intensely red skin 
develops which is not soluble in hot water. 

When CS, acts on collagen in the presence of alkali, the alkali 
first causes an hydrolysis and the CS, then is taken up by the 
resulting products of hydrolytic splitting. This process Ssadikow 
calls “thiohydratation.” The amount of sulphur taken up de- 
pends upon concentration and time of action of the alkali, i.e. 
- upon the degree of hydrolytic splitting. 

Glutin (gelatin), thionylized by 5-10 per cent. solutions of C8, 
in alcohol, ether, benzene, etc., takes up from 0.32 to 0.40 per cent. 


1 Kolloidchem. Beihefte, Vol. 1 (1911). 
12 Loc. cit.; also Kolloid Z. 1, 193 (1907). 


COLLAGEN OR OSSEIN 117 


sulphur (average =0.39 per cent.). The addition product 
usually shows a dark color with alkaline lead acetate, but always 
exhibits a characteristic “erythrin reaction” as follows: Thionyl- 
glutin dried at 110° is heated on a water bath for ten minutes 
with 1 per cent. chloracetic acid. The solution is filtered, cooled, 
and mixed with three volumes of strong alcohol and then neu- 
tralized with ammonia. A precipitate forms immediately or 
after a while, depending on the concentration of the thionyl- 
glutin; and on standing from two to twelve hours this precipitate 
develops a fine pink color, which starts at the top (probably 
because it is due to oxidation) and gradually deepens to 
brown. 

When CS, acts on highly degenerated gelatin the product is 
yellow and has a characteristic odor of mustard oil. The solu- 
tion of this thionylxanthoglutin (thionyl&glutin) in chloracetic 
acid is yellow, and shows the erythrin reaction markedly. This 
erythrin reaction is also shown to some extent by collagen which 
has not been thionylized. ‘‘Glutein” made from the nasal sep- 
tum cartilage of the pig bound from 0.61 to 1.89 per cent. sulphur 
depending on the degree of hydrolysis. 

Glutin brominated in ethereal solution adds as much bromine 
whether thionylized or not. Tanning with tannin or formalde- 
hyde does not interfere either, nor does treatment with methyl 
iodide or benzoyl chloride. 


Chondrigen, Chondrin and Mucin. 


These substances are apt to be met with in glues, but may be 
considered as impurities in the higher grades.1? Chondrigen 
occurs in hyaline cartilage, of which it forms the chief organic 
constituent. It is elastic, semi-transparent, insoluble in hot or 
cold water, and swells but slightly in water or dilute acetic acid. 
- Upon heating for some hours with water under pressure (120°) 
chondrigen yields chondrin, whose solutions gelatinize upon cool- 
ing. ‘Thus chondrigen, as its name indicates, is the parent sub- 
stance of chondrin, just as collagen is the parent substance of 
gelatin. 


143 Mucin is a prolific source of foam in glue. It may be liberated from 
chondrin during manufacture of the glue. 


118 GLUE AND GELATIN 


Allen gives the following procedure for preparing approxi- 
mately pure chondrin: Boil costal cartilage in water for a few 
minutes and after scraping off the perichondrium, boil with 
water at ordinary pressure for 24 hours, or under pressure (at 
120° C.) for 3 to 4 hours. Filter the solution to remove elastin, 
cellular elements, etc., and then precipitate the chondrin with a 
large excess of alcohol, and wash the precipitate with alcohol 
and ether. Re-solution in hot water followed by precipitation 
with alcohol will still further purify it. The chondrin thus pre- 
pared is hard, transparent, odorless, tasteless, and insoluble in 
cold water. Hot water dissolves it, yielding on cooling a jelly 
of weaker strength than that given by the same percentage of 


gelatin. 
Some analyses of chondrin are given herewith: 
6 H N O Ss 
VE MEta cise | One rs ow Vet en oh eaten 49.3 6.6 14.4 29.3 0.4 
Fischer and Bodecker ............. 50.0 6.6 14.4 28.6 0.4 
Schiitzenberger and Bourgeois ..... 50.16 6.58 14.18 29.08 0.0 
Vonsiiehring 2%... v.c. wean ee dee a ee 47.74 6.76 13.87 31.04 0.6 


Allen states: “It will be seen from the above figures that the 
elementary analysis of chondrin present considerable discrep- 
ancies, and suggest that the substance dealt with is not a defi- 
nite substance but is liable to variations in composition. The 
results obtained by Morochowetz, and confirmed by Landwehr, 
Krukenberg, and Morner, strongly support this view. Moro- 
chowetz found that on treating cartilage from various sources 
with lime- or baryta-water, a 0.5 per cent. solution of sodium 
hydroxide, or a 10 per cent. solution of common salt, mucin is 
dissolved out, and may be thrown down from the solution by 
acetic acid; while the substance left undissolved is readily con- 
vertible by boiling water into perfectly normal gelatin. Ac- 
cording to these observations, chondrin is a mixture of gelatin 
and mucin, while chondrigen is a mixture of collagen with mucin 
or hyalogen, the latter component masking its true nature.” 

This led the writer to remark ** that chondrin is probably an 
adsorption compound of simpler substances, ie. gelatin and 
mucin. 

The following table shows that chondrin behaves toward re- 


agents like a mixture of gelatin and mucin: ~ 


“Comm. Organic Analysis,’ 4th ed., Vol. VIII, p. 625. 
15 Allen, loc. cit., p. 626. 


COLLAGEN OR OSSEIN 119 


Gelatin Chondrin Mucin 
RENUDMItY Ys ory... Insoluble in cold 
water, alcohol, or 

ether. Same. Same. 


Soluble in hot 
water; solutions 


gelatinize on 
cooling. Same. Insoluble in hot 
Reaction with let 
Acetic Acid..... No ppt. Ppt.; insoluble ex- 
cept in large ex- 
| cess. Same. 
Mineral acids..... = No ppt, Ppt.; readily solu- 
ble in excess. Same. 
Watnic acid....... Ppt. Ppt. No ppt. 
Mercuric chloride.. Ppt, Ppt. No ppt. 
Lead acetate...... No ppt. Ppt. Ppt: 
Sy ee No ppt. Ppt. Pot. 
Boiling dilute min- 
eral acids....... No reducing sub- 
stance formed. Yield syntonin and 
reducing sub- 
stances. — Same. 


For further information regarding these substances, see Chap- 
ter II, p. 28, under Glycoproteins. 

By microchemical staining methods Morner found that 
tracheal cartilage had a collagenous network enclosing “chondrin 
balls.” By treatment with 0.1 to 0.2 per cent. HCl followed 
by treatment with 0.1 per cent. KOH he dissolved out the ‘‘chon- 
drin balls,’ and then with the aid of dilute acid or superheated 
water converted the network largely into typical gelatin. The 
“chondrin balls” proved to be a mixture of free chondroitic acid, 
and chondromucoid. This latter is the same as the chondromu- 
coid isolated from tendons by Gies and Buerger, and on decom- 
position it yields a protein fraction and chondroitic acid (chon- 
droitin sulphuric acid or cartilage acid), which when acted upon 
by acids gives free sulphuric acid and chondroitin. This latter 
substance yields acetic acid and chondrosin, which according to 
Hawk reduces Fehling’s solution even more strongly than dex- 
trose. 

Upon hydrolysis chondrosin is split into glucosamine and 
glycuronic acid. 

The cartilages, which are precursors of bone, are thus related 


120 GLUE AND GELATIN 


to the mucins, and in fact free sulphuric acid has recently been 
found in the slime of snails. A. P. Mathews? points out the 
importance of this relation from the standpoint of evolution, for 
it shows a chemical relationship of mesodermal to ectodermal 
tissues. Thus chitin which forms the shells of arthropods (e.g. 
beetles) yields on hydrolysis acetic acid and glucosamine, and 
some sulphuric acid is also present. 


16 “Physiological Chemistry,’ 3d ed., 1920. 


Chapter 9. 


The Effect of Tanning Substances on Glue 
and Gelatin. 


According to J. T. Wood! one of the earliest contributions to 
the chemistry of tanning was made by Humphry Davey on 
February 4, 1803, in a paper entitled “An Account of some Ex- 
periments on the Constituent Parts of certain Astringent 
Vegetables, and their Operation in Tanning.” ? Davey remarked: 
“The tanning principle in different vegetables, as will be seen 
hereafter, demands for its saturation different proportions of 
gelatin, and the quantity of the precipitate obtained by filtration 
is not always exactly proportional to the quantities of tannin and 
gelatin in solution, but is influenced by their concentration. 
Thus, I found that 10 grains of isinglass, dissolved in two ounces 
of distilled water, gave with solution of galls in excess, a pre- 
cipitate weighing, when dry, 17 grains, whilst the same quantity 
dissolved in six ounces of water produced, all other circumstances 
being similar, not quite 15 grains. With more diluted solutions, 
the loss was still greater; and analogous effects took place when 
equal portions of the same solution of isinglass were acted upon 
by equal portions of the same infusion of galls diluted in dif- 
ferent degrees with water, the least quantity of precipitate being 
always produced by the least concentrated liquor.” 

The amount of tannin precipitated by 100 parts of gelatin is 
reported by different authors as follows: 


I CR CEE. J's ca ec bc lee cee ss vecepecccevetsawdees — 85 
Lipowitz (Jahresb. Forts. Chem., 1861, p. 624)............-. — 65 
S. Rideal (“Glue and Glue Testing,” 1900, p. 111)......... —134 
ee as oh fs aia a 0b vies ace Vie edb pease w oltnukis —135 
IRM od pct diese wo chan cle) teeeas — 78 
eM ee dn ae a's vis px he's s Wa vce os bs aaln die eaiele cd amen — 50 


One cause for the discrepancies was found by Wood to be 
the fact that a definite excess of tannin is required to produce 


1 J. Soc. Chem. Ind. 27 (1908). 
2 Roy. Soc. London, Phil. Trans. 233 (1803). 


121 


122 GLUE AND GELATIN 


4 maximum amount of precipitate. His results are given in 
the following table, which shows the amount of tannin precipi- 
tated by adding 1 gram of gelatin to varying quantities of 1-100 
tannin solution. 


No. of cc.1% Tannin Tannin pptd. 
Tannin Solution Grams Grams 
100. Fo. 35 oe 45 See oe eee 1 0.91 
200. 6s caeh> vasa abies cere 2 1.50 
OUO Vacs ve oka male hoe oa ee te 3 1.90 
ADO SO, ois a aiae Se os ee ee 4 244 
DOU ares 22a 9 a cee ee eens 5 2.28 
GOO Ook. co haue 3 os ete Berea ee 6 2.36 
LOO RAs EAST Ns pees oe 7 2.36 
BOO fair shew arse eho eran ee eros 8 2.36 


Wood also made a quantitative record of the well-known fact 
that the precipitate of gelatin with excess of tannin has a dif- 
ferent composition from the precipitate of tannin with excess of 
gelatin, and that a considerable amount of tannin may be re- 
moved from the latter by washing with hot water. He found 
that 100 parts of gelatin carried down 300 parts of tannin, of 
which 88 parts were given up upon washing with boiling water.* 

In conclusion Wood observes: “An examination of the facts 
shows that the combination of gelatin and tannin compound is — 
not of constant composition, nor a purely physical one, since 
it does not obey the solution laws, which require the concentra- 

tion of the tannin in the solution and the tannin in the gelatin 
to maintain a constant ratio.” It is therefore an error to con- 
sider “tannate of gelatin” as a definite chemical compound, for 
it is a typical adsorption compound, and as von Schroeder‘ has 
shown, the precipitation of gelatin by tannin follows the adsorp- 
tion isotherm. | 

The precipitation of gelatin by tannin is also a typical in- — 
stance of the precipitation of one colloid by another of opposite 
charge. Aqueous solutions of tannin are positively charged 
(negatively conducted), whereas gelatin is amphoteric, and as 
Ricevuto > has shown is not precipitated by tannin unless in the 
negative condition (positively conducted). Carefully dialyzed 

’ Apparently the excess of gelatin exercises a protective action so that part 
of the tannin, and some of the gelatin as well, are washed away, probably in 
colloidal solution. 


* Kolloidchem. Beihefte 1, 1. 
5 Kolloid Z. 3, 114 (1908). 


THE EFFECT OF TANNING SUBSTANCES 123 


gelatin is not precipitated by tannin, nor is hide tanned by tannin 
unless it is on the acid side. 

When dry the gelatin-tannin compound forms a yellowish- 
brown, brittle mass which melts in boiling water to a tenacious 
sticky mass like bird-lime. In this state it may be drawn out 
or spun into fibers fine as a spider’s web, which have a metallic 
luster like silver slightly tinged with gold. When soaked in alum 
solution they acquire a blue tinge like polished steel. The 
gelatin-tannin compound is tasteless and does not yield tannin 
to alcohol, ether, or acetone. Prolonged boiling with water, 
especially in the presence of magnesia, decomposes it, probably 
because of the hydrolytic cleavage of the gelatin. 


Chrome. 


The chroming of gelatin does not affect its absorption of 
tannin, for a sheet of heavily chromed gelatin absorbs as much 
tannin as before chroming. This fact is not so strange as it 
seems if we remember that the chromium is absorbed only from 
basic solutions, and is apparently attracted to a different part 
of gelatin complex. It seems difficult, however, to reconcile 
with the fact that treatment with basic chromium salts renders 
collagen insoluble; indeed the progress of chrome tannage may 
be followed by immersing strips of the hide in boiling water, 
which causes distortion by converting any unchanged collagen » 
into glue. Chrome tannage appears to be the “mirror picture” 
of tannin tannage, positively charged gelatin being precipitated 
by -negatively charged colloidal chromium hydroxide. This 
view is supported by the experiments of Bancroft,’ who found 
that gelatin sheets took up chromic sulphate practically un- 
changed. If the gelatin containing chromic sulphate is washed 
repeatedly with boiling water, acid is slowly extracted together 
with some gelatin. By treating with dilute alkali, however, 
the acid may be removed without causing swelling or solution 
of the gelatin, which is combined with about 3.3 to 3.5 grams 
of Cr,O, per 100 grams of gelatin. | 

Since Namias showed that the tanning action of chrome alum 


® No attempt will be made here to consider the question of the tanning of 
hides and skins, which is even more complicated than the tanning of gelatin. 
7™“Applied Colloid Chemistry,” p. 229. 


124 GLUE AND GELATIN 


was increased by adding alkali up to the point of precipitation 
of hydrous chromic oxide, Lumiére and Seyewetz made experi- 
ments with the green basic chromic sulphate of Recoura, and 
found it yielded a more insoluble gelatin than did a less basic 
solution. Excess alkali apparently produces such a high degree 
of dispersion of the chromic oxide, that little or no tanning 
occurs. 


Organic Substances. 


Besides tannin and basic chromium solutions, many other sub- 
stances are well known as tanning agents for gelatin; chief 
among them are alums, formaldehyde, and ferric salts. L. 
Meunier and A. Seyewetz ® report having obtained the precipi- 
tation of gelatin solutions with the following organic compounds: 
phenol, resorcin, orcine, hydroquinone, pyrocatechin, gallotannic 
acid, pyrogallic acid, p-amidophenol, chlorophenol, picric acid, 
monochlorhydroquinone (durol), R acid (disulfo-6 naphthol 
2.3.6), G acid (disulfo-B-naphthol 2.6.8.), S acid (monosulfo- 
6-naphthol 2.6.). From their results with various substituted 
quinones ® they conclude that the tanning action of a quinone in- 
creases in rapidity with decreasing power of penetration. The 
importance of this tanning action in the “hardening” or fixing 
of gelatin-coated photographic negatives or positives, must be 
at once manifest. ‘“‘Neredol,”’ the sulphonated phenol-formalde- 
hyde patented product of Stiasny, is largely used to tan leather. 


Bichromates. 


A large literature exists regarding the action of light on gela- 
tin containing bichromates.?° 

The tanning effect, which forms the basis of several photo- 
graphic reproduction and engraving processes, seems to depend 
upon the liberation of colloidal chromic oxide, for as 8S. J. 
Levites 1 has shown K,CrQO, is reduced to Cr,O, by most albu- 
minoids. A. and L. Lumiére and A. Seyewetz?* have shown 

8 Collegium, 1908, No. 318, p. 195. 

® Collegium, 1914, No. 531, p. 528. 

10 See e.g. Eder, ‘‘Reaktionen der Chromsiiure und der Chromate auf organische 
Substanzen in ihren Beziehungen zur Photographie,” 1878. 


1 Kolloid Z. 9,5 (1911). 
12 Phot. Korresp, 6, 75, 192, 239 (1906). 


THE EFFECT OF TANNING SUBSTANCES 125 


that the illuminated bichromate-gelatin differs from that tanned 
by basic chromium salts, the chromium oxide in the former con- 
sisting of two fractions. The first, which equals 3.5 per cent. of 
the bichromated gelatin, represents what is held by the tanned 
gelatin; the second varies with the time of illumination, and 
arises from the reduction of the bichromate by light in the 
presence of organic matter. The first fraction increases dispro- 
portionately to the illumination, and decreases with increasing 
concentration of bichromate. 


Silicic Acid. 

Colloidal silicic acid reacts with gelatin to form a co-silicate 
or colli-silicate of gelatin, whose composition varies with condi- 
tions of its formation. Graham states: 4? “When a solution of 
gelatin was poured into silicic acid in excess, the co-silicate of 
gelatin formed gave, upon analysis, 100 silicic acid with 56 
gelatin, or a little more than half the gelatin stated above as 
found in that compound prepared with the mode of mixing the 


solutions reversed. The gallo-tannate of gelatin is known to 
offer the same variability in composition.” 


Alum. 


The action of alum, and other aluminium salts, in tanning 
gelatin, appears to be consequent upon their hydrolysis, colloidal 
alumina being formed and fixed by the gelatin, while the acid 
may be differentially washed or diffused out. The results of A. 
and L. Lumiére and A. Seyewetz ** are briefly: 

(1) Aluminium salts and nascent alumina raise the setting 
point of gelatin, the effect depending on the percentage of alu- 
mina present. 0.107 grams of Al,(OH), per 100 grams gelatin 
raises the gelatinization point 1°. (2) Alum has relatively a 
_. weak effect, as is to be expected; aluminium chloride has the 
greatest effect. (3) Irrespective of the kind of aluminium salt 
used, the settling point rises with increasing alumina content, 
up to about 0.64 grams alumina per 100 grams gelatin. Over 
this the setting point remains stationary and then falls. (4) The 
increase in the setting point varies with the concentration of the 


1% Phil. Trans. Roy. Soc. London 151, 206 (1861). ; 
1447. fiir wiss. Photographie 4, 360 (1906); Bull. Soc. chim. Paris 35, 676 
(1906). 


126 GLUE AND GELATIN 


gelatin solution. (5) Gelatin fixes a maximum of about 3.6 
grams alumina per 100 grams gelatin, and gives up to the water 
the acid and salts with which the alumina was combined. It 
was concluded, therefore, that gelatin forms a definite chemical 
compound with alumina. 

Lumiére and Seyewetz also found that an excess of alkali or 
ammonia completely inhibited the tanning of gelatin by alumina, 
just as is the case with chrome salts. 

In reviewing this work, H. Freundlich *® expressed the view, 
in which the writer concurs, that it is more probable that a 
colloid complex is formed, rather than a chemical compound 
between hydroxide of aluminium and gelatin. The effects of 
selective absorption and differential diffusion are so great 1° 
that even potassium sulphate may be decomposed by percolat- 
ing its dilute solution through a column of sand, the alkali being 
held by the sand, while a dilute solution of sulphuric acid issues 
from the bottom. 

Gutbier, Sauer, and Schelling ‘” report that at ordinary tem- 
peratures a higher concentration of alum is required to raise the 
viscosity of bone glues than of hide glues. Alum lightens both 
glues, and at higher temperatures, if the solution is slightly acid, 
forms a precipitate, which is an adsorption compound and clari- 
fies better if it settles rapidly. On dialysing glue solutions the 
aluminium only is held back; the colloidal aluminium hydroxide 
and the H-ion concentration control the action of the alum, 
optimum conditions varying with kind of glue and with concen- 
tration. Hide glues seem especially sensitive to the’ action of 
an excess acidity, for with them more often than in bone glues, 
alum clarification causes hydrolysis resulting in foam and de- 
creased strength. The precipitate, however, seems to adsorb 
impurities; it removes part of the ash-producing substances and 
all the addéd acid; and the resulting solution contains but 
little Al. : 


Iron. 
The action of iron on gelatin is well known to gelatin manu- 
facturers, for rusty nets often produce an insoluble reddish- 


18 Kolloid Z. 1, 157 (1906). 
16 See J. Alexander, J. Am. Chem. Soc. 39, 84 (1917). 
% Kolloid Z. 30, 876-95 (1922). 


THE EFFECT OF TANNING SUBSTANCES 127 


brown compound. According to Liippo-Cramer ** ferric chloride 
solutions precipitate gelatin. A 1 per cent. solution of gelatin 
mixes with the chloride without precipitation, but the color is 
darker than a solution of ferric chloride of the same concentra- 
tion, showing that the salt undergoes, in the presence of gelatin, 
an hydrolysis similar to that which it suffers on boiling. Iron 
alum also tans gelatin, but. not in the presence of an excess of 
alkali, which turns the gelatin dark but allows the iron to be 
washed out. The adsorbed ferric hydroxide can also be washed 
out by potassium citrate or oxalate, and by oxalic and other 
acids. Fifty cc. of 10 per cent. FeCl, + 50 cc. 10 per cent. gela- 
tin solution at 50° give a thick red-brown fluid which sets and 
can be remelted. Here the excess of gelatin evidently acts as a 
protective colloid to the iron-gelatin adsorption compound. In 
fact, according to Stiasny the unsatisfactory tanning action of 
iron-salts consequent upon their rapid and complete hydrolysis, 
is improved by the presence of soap, blood, albumen, gelatin, and 
similar colloidal protectors. 


Other Salts. 


Uranic salts (e.g. uranium nitrate) act similarly to ferric salts, 
and auric chloride has a particularly powerful tanning action, 
which it likewise exerts on the skin. Copper, silver, mercury 
and lead salts are powerfully fixed by gelatin, and even barium 
chloride undergoes a partial hydrolysis in its presence. Indeed, 
as Van Bemmelen’® has shown, colloids by their adsorptive 
action can effect a chemical decomposition of most salts. Thus 
if a red solution of thiocyanate of iron is added drop by drop 
to a 10 per cent. solution of gelatin, the ruby-red precipitate 
soon changes to the rust-brown color of ferric hydroxide. . 

Phosphomolybdic and phosphotungstic acids precipitate gela- 
tin, and its precipitate with picric acid is used for its detection 
in the Stokes method.?° 


The Halogens. 


The halogens have a powerful tanning action on gelatin. As 
far back as 1840, Mulder ** described the compound formed by 

18 Kolloid Z. 1, 353 (1907). 

19 Rec. Trav. chim. ‘Pays Bas 7, 37 (1888). 


20 Analyst, 1907, p. 320. 
21 Berzelius Jahresber. 19, 734; J. fiir Chem. 44, 489. 


128 GLUE AND GELATIN 


treating gelatin with chlorine, and Allen and Searle ?? described 
a similar compound with bromine, while Hopkins and Brooks ** 
made like observation with respect to iodine. 

According to Rideal and Stewart 24 when chlorine is bubbled 
through 1 per cent. gelatin solution the liquid remains clear for a 
time and then froths, each bubble of gas becoming encased in a 
white pellicle. With an excess of chlorine the liquid becomes 
clear again and the gelatin forms a white granular precipitate 
which on washing and drying yields a pale yellowish-white pow- 
der, odorless, tasteless, and insoluble in water or alcohol, but 
soluble in alkalis. | 

Cross, Bevan, and Briggs ?> found that moist gelatin spread 
out very thin by immersing cotton yarn in its solution, combines 
with 15.4 per cent. of chlorine figured on air-dried gelatin. The © 
extremely stable substance resulting they regard as a gelatin 
chloramine; it is sensitive to antichlors, and when treated with 
sulphuric acid reverts to the original gelatin. This reaction is 
made the basis of a method for the detection and estimation of 
gelatin in tub-sized papers (loc. cit., p. 263). — 

Lumiere and Seyewetz 7° found that the best results with chlo- 
rine were obtained by adding, say, 10 grams of gelatin to 500 ce. 
of a saturated solution of chlorine, containing 50 grams of NaCl 
and held at 0° to drive back the ionization of hydrochloric acid. 
They found it much easier to use hypochlorites, 10 grams of gela- 
tin in thin sheets being rendered insoluble at room temperature 
by a solution of 100 grams of commercial sodium hypochlorite - 
and 2 cc. of HCl (21° Bé) in 400 cc. of water. They found that 
bromine acted similarly but more energetically, but were unable 
to render gelatin insoluble with iodine. 


Formaldehyde. 


The tanning action of formaldehyde, both in solution and as , 
a gas, has long been known and utilized. Acrylic aldehyde is 
said to act similarly, but acetic aldehyde acts only in the pres- 
ence of water. 

2 Analyst, 1887, p. 258. 

J, Physiol. 22, 184. 

24 Analyst, 1897, p. 228. 


25 J. Soc. Chem. Ind. 27, 260 (1908). 
2° Bull. Soc. Chim. (4) 11, 344 (1912). 


THE EFFECT OF TANNING SUBSTANCES 129 


The maximum amount of formaldehyde fixed, when its 10 per 
* cent. solution acts on dry gelatin, is between 4.0 and 4.8 per 
cent.27 The “insoluble” formo-gelatin is decomposed by re- 
peated washing with boiling water, as well as by heating to 110°, 
and by cold 15 per cent. HCl. R. Abegg and P. von Schroeder 7° 
half filled a test tube with a 10 per cent. solution of gelatin 
which had a melting point of 36°, and after the gelatin had set, 
covered it with a 5 per cent. formalin solution which was allowed 
to act for 24 hours. The upper fully tanned layer was infusible, 
but crumpled at 85° with the development of a brown color. 
Lower layers showed melting points of 48°, 42°, and 37°, whereas 
the bottom layer was unaffected. They also found that the time 
of tanning (as determined by the time needed to reach the high- 
est melting point observed, 48°) varied inversely with the con- 
centration of the formaldehyde. 

H. Bechhold?® prepares ultra filters by impregnating filter 
paper with gelatin solutions of various strengths, allowing the 
gelatin to set, and then immersing the treated paper in 2-4 per 
cent. ice-cold formaldehyde. 

R. H. Bogue *° has examined into the effect of formaldehyde 
on various glues. He found that the viscosity increased directly 
as the amount of formaldehyde added; it decreases with rise of 
temperature up to 40°, after which it rises rapidly to the setting 
point. It also increases with time. On the other hand the jelly 
strength of glues is decreased proportionately to the amount of 
formaldehyde added, the effect being most marked in lower con- 
centrations and with weaker glues, some of which actually re- 
mained fluid. The higher the grade of glue and the higher its 
concentration, the less formaldehyde is required to produce “‘in- 
solubility.” Alums produce increased viscosities but have little 
or no effect on the jelly strength.*? 

7 Lumiére and Seyewetz, Bull. Soc. Chim. 35, 872 (1906). 

28 Kolloid Z. 2, 85 (1907). 

22“Colloids in Biology and Medicine,” p. 97. 


80 Chem. Met. Hng. 23, 61 et seq. (1920). 
31 Chrome alum increases the melting point however. 


Chapter 10. 


The Chemical Examination of Glue and Gelatin. 


Hydrogen Ion Concentration, or pj, Value. 


Since recent investigations + have shown the great influence of 
the effective reaction (hydrogen ion concentration or p,, value) 


on the viscosity and jelly strength of glue and gelatin, its deter- 
mination by the electrometric or the colorimetric methods may 
form a necessary part of factory control.2 For colorimetric 
determination of p,,, the following indicators are used: 


Color change 


Indicator Py range acid-alkaline Solution strength 

Methyl Violet ........ O1to 32 green to blue 0.02% 
Thymol Blue .......... 12to 28 red to yellow 0.04% 

(lower range) 
Methyl Orange ....... 3.2to 44 red to yellow 0.02% 
Methyicdveds ue ee 44to 6.0 red to yellow 0.02% in 60% alcohol 
Fheucl Hedicse. ese 68to 84 yellowtored 0.02% 
Thymol Blue ......... 8.0to 96 yellow to blue 0.04% 

(upper range) 
Phenolphthalien ...... 8.3to10.0 colorless to red 0.05% in 50% alcohol 


The limitations of these indicators for gelatin are still to be 
determined, and in all cases the percentage ‘of gelatin present is 
a factor of importance. That is, a one per cent. solution may 
show a different p,, than a 5 per cent. solution. Patten and 


Johnson ** state that gelatin does not interfere with the deter- 
mination of Re colorimetrically. 


Bogue even recommended the determination of H-ion concen- — 
tration as part of the regular laboratory routine test of both glue 
and gelatin. He states*: “If the p,, value is 4.7, the viscosity, 


swelling, etc., are low, and the product nearly insoluble. On 
either side of this point * these properties increase very consid- 


1J. Loeb, R. H. Bogue and others. 

* For details see W. M. Clark, ‘‘The Determination of Hydrogen Ions,” Balti- 
more, 1920. 

2a J. Biol. Chem. 38, 179 (1919). 

3.7. Ind. Hng. Chem. 14, 439 (1922). 

*This is the isoelectric point of gelatin. 


130 


CHEMICAL EXAMINATION OF GLUE AND GELATIN 131 


erably, attaining their maximum on the acid side at Py, 3-5, and 
on the alkaline side at P,, 9-0. At greater acidity than p,, 3.5 
or at greater alkalinity than P,, 9.0 these properties again de- 
crease. The ee value indicates, therefore, not only the reaction 


of the material, and the degree of acidity or alkalinity, but also 
the proximity of the substance to the points of maximum or 
minimum properties. ... One per cent. solutions are best in 
either case, and the results expressed in terms of P,; to the 


nearest tenth.” 

The experiments of Bogue and of Loeb were made with rather 
dilute solutions of glue gelatin. In actual practice the relatively 
small variations in the H-ion concentration of glues produce such 
slight changes in the viscosity of working solutions that they 
are barely perceptible with the viscosimeters in common use. 

Outside of moisture and ash which have already. been con- . 
sidered, chemical tests on glue are seldom made, unless they be 
tests to simulate working conditions where alum, formaldehyde 
or other substances are added to the glue. The reason is that 
most of those whose knowledge of. glue transcends laboratory 
experience, are agreed that the chemical tests proposed for 
glue do not enable one to form any trustworthy idea as to its 
practical value, and they are besides much more difficult, expen- 
sive, and time-consuming than the very satisfactory physical 
tests. In the case of food gelatin it is essential to make an exact 
estimation by chemical methods of arsenic, zinc, copper, lead, 
sulphur dioxide and ash. Chemical tests are required for gelatin 
intended for special uses, e.g. photography (see Chapter 18, 
p: 208). 

Acidity or alkalinity may be determined by titration. To 
estimate free acid, Kissling® soaks 30 grams of glue in 80 cc. 
of water for several hours and then drives over the volatile acid 
by a current of steam. When the distillate amounts to 200 cc., 
it is titrated with standard alkali, and titrate back the unused 
portion. 


Total Acidity. 


The total acidity of a glue may, of course, be determined 
directly by titration with 0.1 N NaOH solution, using phenol- 


5 Chem. Z. 11, 691. 


132, GLUE AND GELATIN 


phthalein or roseolic acid as an indicator. H,SO, may then be 
determined by a separate titration with 0.1 N I solution. For 
more accurate work phenol red (effective range p,,= 6.8 to 8.4) 


should be used as indicator. If the glue contains formaldehyde 
or other substances which react with iodine, H,SO, must be 
determined by acidifying with H,PO,, distilling off the SO, in a 
current of steam or CO,, and weighing it as BaSO,. In the case 
of bone glues a direct titration of the acids other than H,SO, 
may be made with 0.1 N NaOH, using as an indicator alizarin 
which, in the presence of at least 1 per cent. of glue, possesses the 
curious property of reacting only with strong acids and not with 
H,SO,.° Owing to legal restrictions the exact estimation of SO, 
and sulphates in gelatin is of great importance and will be 
referred to later. 


Determinations Involving Nitrogen. 


Clayton* considered the estimation of non-gelatinous sub- 
stances the best single chemical test for glue. To determine non- 
gelatinous substances, C. Stelling * dissolved 15 grams of glue 
in 60 ce. of water made up to 250 ec. with 96 per cent. alcohol, 
and after thorough shaking, evaporated to dryness 25-50 cc. of 
the fluid filtered off after standing six hours. Trotman and 
- Hackford ® separated the hydrolyzed from the’ non-hydrolyzed 
products by precipitating the former by saturation with zine 
sulphate and estimating them by the Kjeldahl method. H. J. 
Watson ?° does not regard the test as having any value. 

R. H. Bogue ** found that the ash and total nitrogen bore no 
consistent relation to the jelly strength of glues, but that the 
strongest glues showed the highest moisture content. Bogue 
made the following determinations on a series of hide and bone — 
glues and a few other glue products: ™ 


6 Gutbier, Sauer and Brintzinger, Kolloid Z. 29, 180 (1921). 

7J. Soc. Chem. Ind. 21, 670 (1902). 

8 Chem. Z. 20, 461; Analyst 21, 289 (1896). 

®J. Soc. Chem. Ind. 24, 1072 (1904). 

10 J, Soc. Chem. Ind. 23, 1189 (1904). 

11 Chem. Met. Eng. 23, 61 et seq. (1920). 

2 For details regarding these methods see S. B. Schryver, ‘“‘Allen’s Comm. 
Organic Analysis,’ 4th ed., Vol. 8, pp. 467 et seq. Schryver recommends the 
addition of 2 ce. of diluted sulphuric acid (1 part concentrated acid to 4 parts 
water to each 100 ec. of mixed protein and sulphate solutions) for the protein 
precipitation, for which he used zine sulphate. Bogue, using magnesium sul- 


ee found maximum precipitation to occur with % ec. of dilute sulphuric 
acid. 


CHEMICAL EXAMINATION OF GLUE AND GELATIN 133 


(1) Total nitrogen, by Kjeldahl’s method. 

(2) Protein nitrogen, from the precipitate formed on adding 50 
cc. saturated magnesium sulphate solution to 50 cc. of 
water containing one gram of glue. 

(3) Protewn-proteose nitrogen, from the precipitate formed by 
saturating a similar glue solution with magnesium sul- 
phate. 

(4) Proteose nitrogen, difference between (3) and (2). 

(5) Amino mtrogen, from the filtrate of (3), using Sdrensen’s 
formaldehyde titration method. 


Bogue found that the temperature exercises a marked influence 
on protein precipitation, 3 to 8 per cent. more coming down at 
17° than at 25°, but his determinations were made at 25° as 
this was more convenient.?® 

Bogue’s results are given in the following table: 


RELATION BETWEEN NITROGENOUS CONSTITUENTS AND JELL STRENGTH 


Amino 
Protein Proteose Peptone Acid 
Grade N N Nd N 
H, 92.2 6.3 1.1 0.4 
H2 90.4 7.0 2.0 0.6 
H; 86.2 12.0 14 0.4 
Hide and fleshing Hy 84.6 12.4 26 0.4 
ies ss » H; 78.7 16.0 45 0.8 
H, 776 17.0 4.7 0.7 
Hs 52.0 38.6 8.4 0.9 
Bi 79.1 14.9 48 1.2 
Bz 185 16.4 8.1 2.0 
Bs 64.6 28.3 5.6 1.5 
Bz 59.8 32.4 6.4 14 
Bone glues ........ Bs 53.6 36.6 84 14 
Bs 52.5 37.9 78 18 
B; 48.2 40.1 10.1 16 
Bs 36.8 47.1 12.5 2.3 
9 31.5 50.6 14.8 3.0 
Special Glues 
Russian isinglass.... He 91.0 44 4.5 0.1 
Edible gelatin ..... Hz 87.8 11.3 0.7 0.2 
Piso Ciie’...+>..-... Bo 33.4 42.3 21.9 24 
Pressure tankage ... Bz 34.3 46.4 16.3 3.0 
CG Bs 0.0 33.2 48.5 18.3 


(Here 1 represents the highest grade and 9 the lowest.) 


13 Samples secured by empirical precipitation methods of this character, ob- 
viously represent mixtures of substances of variable composition. 


134 GLUE AND GELATIN 


These figures indicate that the jelly strength varies approxi- 
mately as the protein nitrogen determined by Bogue’s procedure. 
No consistent relation could be shown, however, between viscosity 
and nitrogenous constituents. The amino nitrogen is greater in 
bone than in hide glues, and tends to increase with decrease in 
jelly strength. 

Bogue also treated a number of glues having uniform jelly 
strength (grade), but different viscosities, with various concen- 
trations of magnesium sulphate from 50 per cent. down to 24 
per cent. of saturation. Below that the precipitate was so finely 
subdivided and slimy that filtration was practically impossible. 
The results, given in the following table, show that there is no 
definite relation between viscosity and precipitate, except that 


SHowina Per Cent. or NitroceEN THRowN Down By VARYING PERCENTAGE 
SATURATIONS OF MAGNESIUM SULPHATE 


50 36 30 28 26 24 
Per Per Per Per Per Per 
No. Grade Jell Visc. Cent. Cent. Cent. Cent. Cent. Cent. 


Series 8...... 1 H. 65 456 844 692 453 — — — 
2 HH, 65 472 873 G88 e4Gn — — — 
3. Hy 66° 480. 870° -(saeeeee — — — 
4°- Hy. 66 49.0" 852) \7i pee — — — 
5 HH, 66 502. -852 68.005 — 346 286 
6 H, 64 510. 849° GS0>saZz2 — 35.3 290 
7 HH, 64° 540 S818. 645835503 — 348 30.0 
RBs? 68924435 25 ioe — 22.3 — mas = 
Bs 3 2 OR 4a Rou — 36.3 — — — 

10>). By ee Ue eee — 38.9 a — — 

il. By 705, 480). 762 — , 398 — — —_ 
50 35 30 28 26 24 

‘Per Per Per Per Pers ree 

No. Grade Jell Visc. Cent. Cent. Cent. Cent. Cent. Cent. 

Series 4 ..... 1‘ Hs 64 45.4 818° 582 44355. — — 
2 Hs 64 474 873 688 460 421 —_ — 
3 Hs 64 506 740 .606 499 S458 — — 
4 HH, 65 4462 854 629 473559905 — — 
5 H, 65° 478 S872 65.1 46 — — 
6 H, 65 480 756 576 44:7 ae — — 
’ HH, 65 494 765 590 “4700 — — 
8 H, 65% 490 722 590 445 42.0 — — 
9 H, 65% 500 860 669 528 44.0 — — 

10 Hy, 65% 502 852 664 521 487 — — 
11 H+ 66 474 830 63.1 47,7 5a — — 
12 Hit 66 486 870 (718) 40 7 — — 
13 Ht 66 488 852 716 503 461 — — 
14 Ht 66 492 862 643 522 468 — — 
15 H.t 66 496 852 680° 517e= eee — — 
146 Ht 66 498 8382 655 52.7 494 — —_ 


CHEMICAL EXAMINATION OF GLUE AND GELATIN 135 


with about 30 per cent. saturation the precipitate varies as the 
viscosity. Bogue interprets these figures to mean that “if the 
jell strength be constant the viscosity will vary as the size of the 
protein molecule.” By the “molecule” he means ‘“‘a group which 
may not be subdivided except by chemical processes, as of hydrol- 
ysis, whereas the colloidal complex is established probably by 
electrical phenomena and the processes chemical condensation 
or hydrolysis are not involved.” 

Bogue also treated several glues having about the same vis- 
cosity, but varying jelly strengths, with 50 per cent. and 30 per 
cent. saturated magnesium sulphate. The precipitates showed 
the following percentage of nitrogen (Kjeldahl): 


50 S30 ; 

No. Jell. Viscosity PerCent. Per Cent. 
RE Sas sa oles « 63 46.2 83.5 45.0 
eee 64 45.8 84.2 46.2 
Oo nee 66 46.0 84.9 46.7 
0 LS 68 458 85.3 65:1 
oS Nee eee 70 46.2 85.5 57.4 


“Tt will be seen that at both 50.0 per cent. and 30.0 per cent. 
magnesium sulphate saturations, the nitrogen thrown down in 
the several precipitates varies directly as the jell strength, the 
viscosities being practically constant. This means then that 
at constant viscosity the jell strength will vary as the size of the 
protein molecule, as well as with the total amount of protein.” 

In the precipitation of purely empirical groups such as “gela- 
tin,” “gelatoses,’ and “gelatones” the larger molecular groups 
are for the most part thrown down more readily: but no quan- 
titative relations are deducible from the amount of the precipi- 
tate, for the various fractions exercise a varying protective 
influence on each other, which accounts for the observations of 
Haslam ** that part of any fraction remains in solution, while 
the precipitate may carry down part of a subsequent fraction.’® 
With precipitates of this character we are obviously dealing 
with molecular groups rather than with simple molecules. 

Upon investigating the crazing of glues, Bogue reports that 
this crackling up of the glue pieces is “due to an exceptionally 
great hydrolysis of the protein molecule and the consequent 


14 J, Physiol. 32, 267 (1905) ; ibid., 36, 164 (1907). 
13 The work of BH. Zunz shows the great variation in the protective action of 
various albumose fractions. 


136 GLUE AND GELATIN 


inability of the resulting mixture to retain water above that 
minimum content below which crazing occurs.” His analytical 
results hardly justify this conclusion, for 7 crazed glues showed 
an average of 11.99 per cent. moisture, while with 7 firm glues 
of equal grade the average moisture was 11.91 per cent. Al- 
though in the firm glues the average protein nitrogen was higher 
and the average proteose and peptone nitrogen lower than in 
the crazed glue, still one crazed glue had the highest protein 
and the lowest proteose and peptone nitrogen. It is true that 
only very low-grade glues craze, but some other factors must be 
reckoned with, probably differences inherent in the original raw 
material, for glue is no more a definite chemical entity than is 
gelatin. Presence of an excess of a fraction having excessive 
syneresis, or diminution of a fraction having protective action 
against a syneresis, might possibly account for crazing. 


Diffusible Nitrogen Test. 


' The British Adhesives Committee *** evolved this test, which, 
they say supplies an indication of the stability of glues towards 
water, and furnishes a rough measure of their tensile strengths, 
the stronger glues generally being low in diffusible nitrogen. 

The glue under test is made up into a jelly containing 2.1 
grams of nitrogen in 75 cc. of water; this requires 15 grams of 
glue, approximately. The exact amount of glue is soaked over 
night in 75 cc. of water, heated to 37° C. for 2 hours, then to 
about 90° C. for 30 minutes (This procedure must produce 
marked hydrolysis. J. A.), and finally poured into a Petri dish 
14 cm. in diameter, where it is allowed to set. One hundred cc. 
of water are now layered over the jelly and the dish is placed 
in a thermostat at 20° C. for 20 hours. The number of milli- 
grams of nitrogen per 100 cc. of the supernatant fluid, as deter- 
mined by Kjeldahl’s method, constitutes the diffusible nitrogen 
number. oe 

Since some of the constituents of the glue act as protectors to 
others and thus tend to peptize them (the Report even mentions 
this on p. 29), and since the value of the various hydrolysis 
products in this respect is not known, the investigation, to use 
the Committee’s own words, is ‘admittedly incomplete, and the 


a Wirst Report, p. 20 et seq., London, 1922. 


CHEMICAL EXAMINATION OF GLUE AND GELATIN 137 


subject requires further study. Again, the test may lend itself 
to the study of the size of the hydrated gelatin aggregate under 
various conditions. The addition to a glue solution of salts that 
lower the surface tension of water will tend to reduce the size 
of the gelatin aggregate, and presumably, consequently, to in- 
crease the amount of diffusable nitrogen.” The test should also 
indicate the degree of hydrolysis. | 

The Committee tested the effect on joint or tensile strength 
and jelly strength of a series of sodium salts of organic acids, 
and also the effect of some sugars. Variations in viscosity com- 
plicate the joint tests, but sodium formate and salicylate ‘and 
the sugars markedly increase the joint strength. 


Reactions of Gelatin. *® 

Gelatin is totally insoluble in absolute alcohol, ether, chloro- 
form, benzene, carbon disulphide, and fixed and volatile oils. 
It is practically insoluble in ice-cold 10 per cent. alcohol, and 
precipitates as a white coherent, elastic mass if an excess of 
alcohol is added to its aqueous solution. The precipitate swells 
in cold water and may be redissolved as before. 

Fairbrother and Swan? tested the “solubility of gelatin in 
cold water, and report the following results given in grams per 
100 cc.: 0.02 at 0°, 0.07 at 18.3°, and 0.10 at 22°. The solubility 
in solutions of hydrochloric, sulphuric, nitric and acetic acids and 
‘in solutions of potassium and of sodium hydrates (concentra- 
tions varying from 0.2 to 5000 millimols per liter) were also 
determined. With acids the solubility passes through a mini- 
mum, rising then to about 0.2, after which solution gradually © 
occurred. With alkalis similar results were obtained, but there 
was no minimum. Neutral salts decreased the solubility, their 
effect being approximately in the order of the Hofmeister series. 

These experiments were done with Coignet’s Gelatin Extra 
analyzing 2.24 per cent. ash and having a P ,, Value in 1 per cent. 


solution of 5.6 at 20°. They should be repeated with ash-free 
gelatin properly freed from products of hydrolysis, for it is 
probable that only the hydrolysis products dissolve in cold 


water. 


16 See also Chapter 9. Most of the reactions apply to the more or less 
impure gelatin known as glue. 
isa FY, Fairbrother and E. Swan, J. CCH. Soc. 121, 1273-44 (1922). — 


138 GLUE AND GELATIN 


According to Zlobicki1® 0.5-0.8 grams of gelatin to 100 ce. 
of water causes a marked lowering of the surface tension of 
water, although further addition does not increase the effect. 

As has been pointed out by Victor Lehner ™ selenium oxychlo- 
ride readily dissolves glue and gelatin in the cold. This remark- 
able solvent likewise dissolves resins (natural and synthetic), 
rubber, shellacs, and asphalt. 

Solutions of gelatin in strong acetic acid do not gelatinize on 
cooling and are used as liquid glues. Warming with dilute 
nitric acid yields a liquid product, but strong nitric acid destroys 
gelatin, giving oxalic-acid and other substances. Gelatin may 
also be held in fluid condition by urea, zinc, magnesium, and 
calcium chlorides, sodium iodide and sodium and calcium nitrates, 
naphthalene sulphonate, etc. 

Gelatin is completely precipitated by saturating its aqueous 
solution with ammonium, zinc, or magnesium sulphates. Phos- 
phomolybdic and phosphotungstic acids also precipitate it, as 
‘do tannin and sufficient quantities of mercuric chloride and of 
picric acid. The reaction with tannin is used to detect gelatin 
(or vice versa), a 0.02 per cent. solution of gelatin yielding a 
white or buff-colored precipitate which is insoluble in presence 
of an excess of tannin. Without such excess the precipitate 
tends to dissolve in pure water, especially if hot, apparently 
going into colloidal solution because an excess of gelatin acts as 
a protector or dispersing agent toward the tannin precipitate. 

Ruffin '* proposed to determine gelatin by precipitating it with 
tannin, and titrating the excess of tannin with iodine. 

The evidence shows, however, that the so-called “tannate of 
gelatin” is not a substance of definite composition. Thus H. 
Trunkel '® found that 1 gram of freshly dissolved gelatin is pre- 
cipitated by 0.7 grams tannin, but after standing 24 hours 0.4 ~ 
tannin will precipitate it. On rewarming the original condition 
returns. Any excess of tannin up to 3 parts per unit of gelatin 
is carried down, but upon washing the precipitate with alcohol, 
97 per cent. of the tannin may be removed. Trunkel’s conclu- 
sion is that the gelatin tannin complex is an adsorption com- 
pound. ° 


1b Bull. Acad. Sci. Cracovie, 1906, p. 497. 
17 J, Am. Chem. Soc. 48, 29 (1921). 

18 Chem. Z. 24, 567 (1900). 

19 Biochem. Z. 26, 458 (1910). 


CHEMICAL EXAMINATION OF GLUE AND GELATIN 139 


J. T. Wood ”° quotes much of the prior work on compounds of 
tannin and gelatin, going back to experiments of Humphry 
Davy,* who found that variable quantities of tannin were fixed 
by isinglass. Wood, using Coignet’s Gold Label Gelatin, found 
that the greatest amount of tannin which could be precipitated 
by 1 gram of air-dry gelatin was about 2.4 grams from a solu- 
tion containing about 6 grams of tannin, the volume of the 
solution after the addition of gelatin being 150 cc. Chromed 
gelatin absorbs just as much tannin as unchromed gelatin. 
Chromed hide powder is used for the assay of tanning materials. 
See Chapter 9 for the action of tanning BONERS: on cee 
and Chapter 8 for the effect of CS,. 

The halogens, chlorine, bromine and iodine react with Pane 
yielding insoluble compounds which form the basis of analytical 
methods. The chlorine compound described by Allen”? is a 
pale, yellowish-white powder, which is odorless, tasteless and 
imputrescible, and insoluble in water or alcohol, but soluble in 
alkalis. Yet Allen found that the bromine precipitation method 
could not be applied to commercial gelatin and glue, which 
‘‘vielded results which at present are incapable of interpretation. 
The completeness of the precipitation of gelatin by bromine- 
water is affected by conditions not at present understood. In 
some cases the precipitation was very complete, while in other 
experiments, in which the conditions were but very slightly 
varied, much nitrogen remained unprecipitated.” 

Since glues and gelatins may vary considerably in composition 
and in their protective action toward the bromine compound, and 
since their “previous history” may affect their ability to form 
adsorption compounds, it is not surprising that Allen reported 
confusing results. | 

Platinic chloride and sulphate give precipitates with gelatin, 
and Crismer recommends an acid solution of chromic acid as a 
precipitant. 

Northrup ** followed the hydrolysis of gelatin by pepsin, 
trypsin, acid, and alkali, and found that the early action of the 
enzymes and alkali were similar but different from the action of 


20 J. Soc. Chem. Ind. 25, 384 (1908). 

2 Phil. Trans. 1808, p. 283. 

2 “Comm. Organic Analysis,” 4th ed., Vol. 8, p. 591. 
23 J. H. Northrup, J. Gen. Physiol. 4, 7 (1921). 


140 GLUE AND GELATIN 


acid. Comparing the relative velocities of hydrolysis of dif- 
ferent peptide linkages, he found that trypsin would attack all 
those linkages that pepsin attacked, and some others besides. 
Linkages rapidly attacked by pepsin yielded only slowly to 
trypsin, while those most rapidly attacked by the enzymes 
yielded readily to alkali but slowly to acid. 

For a discussion of the cleavage products of gelatin, including 
those resulting from bacterial decomposition, see ‘Allen’s Com- 
mercial Organic Analysis,” 4th ed., Vol. 8, pp. 594 et seq. 

According to Seemann.** oxidising agents, like permanganates, 
yield with gelatin such products as: oxalan, NH,.CO.NH.- 
C,O,NH,; ammonium oxaminate, C,0,.NH,.NH,; ammonium 
oxalate; and oxalic, succinic, benzoic, butyric, acetic, and formic 
acids. Sometimes benzaldehyde, propionic and valerianic acids 
are produced. 3 | 

Detection of Glue and Gelatin. According to Allen® the 
property of gelatinizing on cooling is the only test from which 
the presence of gelatin in a complex animal liquid.can be safely 
inferred. To detect gelatin in ice-cream or in cream, the U. 8. 
Department of Agriculture use Stokes’ picrie acid method,?® 
which is as follows: Dissolve 5 grams of mercury in 10 grams 
of nitric acid (sp. gr. 1.42), and dilute to 25 times its bulk. To 
10 ce. of this solution add 10 cc. of cream and 20 cc. of water, 
in order to precipitate all proteins except gelatin. If gelatin 
be present, the filtrate will give an immediate yellow precipitate 
with an equal part of a saturated aqueous solution of picric 
acid. The nitrate solution should give no turbidity with the 
picric acid solution. This test will detect one part of gelatin 
in 10,000 parts of water. 

To detect: gelatin in preserves, A. Desmouliére 27 takes 20 
grams of the sample and precipitates the gelatin by gradually 
adding 100 ce. of 90 per cent. alcohol.27* After standing 2-3 
hours, the supernatant fluid is decanted, the residue dissolved in 
hot water, and tested with picric acid and tannin which give pre- 


24 Zentr. Physiol. 18, 285 41904). 

°° “Comm. Organic Analysis,’ 4th ed., Vol. 8, p. 592. 

26 Analyst, 1907, p. 320. 

7 Ann. Chim. anal. appl. 7, 201 (1902). 

“7a Gelatin is somewhat soluble in 90 per cent. alcohol, and the preserve 
usually contains water; therefore stronger alcohol should be used. Since 
gelatin precipitates most readily at the isoelectric point, the solution should 
have a py value of about 4.7. 


CHEMICAL EXAMINATION OF GLUE AND GELATIN 141 


cipitates in the presence of gelatin. A confirmatory test 1s to 
add quick-lime, which evolves ammonia. 3 

Henzold 28 proposed the following method for detecting gelatin. 
in foods: The specimen is boiled with water and the filtrate 
boiled with an excess of 10 per cent. potassium dichromate. If 
gelatin be present, a few drops of concentrated sulphuric acid 
produces a white flocculent precipitate, which gradually ag- 
glomerates at the bottom of the vessel. 

E. Schmidt 7° makes a reagent said to be sensitive for glue in 
the presence of ammonia, by acidifying Nessler’s reagent slightly 
with sulphuric acid. This gives a red precipitate which is 
filtered off, leaving a clear yellow solution which constitutes the 
reagent. 

Diirbeck *° uses a solution of thionin (a thiazin dye) to detect 
gelatin or agar in sausage. Agar gives a violet color, gelatin 
a deep blue. . 


Gold Number. 


Glue and gelatin possess such a powerful action as colloidal 
protectors, that a determination of the “gold number” after 
Zsigmondy’s method *! may demonstrate their presence, at least 
differentially. The “gold number” is the number of milligrams 
of a colloidal substance which just fails to prevent the color 
change (from bright red to violet) of 10 cc. of a colloidal gold 
solution upon the addition of 1 cc. of 10 per cent. NaCl. The 
colloidal gold solution is prepared as follows: 

One hundred and twenty ce. of distilled water, condensed in a 
silver worm, are placed in a 300-500 cc. Jena glass beaker. While 
heating there are added 2.5 cc. of a 0.6 per cent. solution of gold 
hydrogen chloride and 3-38.5 cc. of 0.18 N potassium carbonate 
solution, both of the highest purity. After boiling add promptly 
3.5 cc. of dilute formaldehyde (0.3 cc. of commercial 40 per cent. 
formal to 100 cc. water). A bright red color should develop 
slowly, usually beginning as a brown or orange tint. Jena or 
equivalent glass should be used throughout, both in preparing 
the solutions and in making tests with them. 


2 Z. offentl. Chem. 6, 292 (1900). 

22 Farber Z. 24, 97. 

30 Z, Nahr. und Geniissm. 27, 801. 

31“Colloids and the Ultramicroscope,” New York, 1909; Z. anal. .Chem. 40, 
697 (1901). 


142 GLUE AND GELATIN 


The following table gives the gold numbers of some proteins 
according to Zsigmondy and Schryver: 


Substance Gold Number 
Gelatin acy oo ace een Ge ks cs el ee ee eee 0.005-0.01 
Russian glue sic. c's sais su ce eee met eee ae 0.005-0.01 . 
[sitiglass’) os SON Oss ae ee re a 0.01 -0.02 
Casein: (in ammonia) 0) ose agewe ss meee Prey rt 
Heg-globulin . . cass cscs <a hereto ran teen 0.02 -0.05 
Ovomucoid 1. fe oo eos ake os eee toe eee 0.04 -—0.08 
Glycoprotein ©. 650 s-iS nse pie cae 0.05 -0.1 
Amorphous egg-albumen ..........5i<-sbeeen 0.03 —0.06 
Crystallized egg-albumen *..%... -.. te. ene 2.0 -8.0 
Fresh egg white...........s000- bo MRss catews esr 0.08 -0.15 
G bi §0.15 0.25 

hiss Weta: |i | Gere er aa REISE ee ko 105 ah 
Gum :tregacanth ../. 5.44 sacas noo tee ee 2.04 
DDOXETIN iy avis 0 3s 7 esis Ue sig etn sd belek ave Cee 6.0 —20.0 
Wheat starch “So o0 icone es 6 ees ce ee 5.0 = 
Potatota Siw. .% Wish li ieatauins aa ee 25.0 + 
BOGHIM: Oleate ous, och eee ie PERI Te ede - 04 -10 
Sodium stearate‘at 100°C... 5c 225 eae ee » 0.01 

as 5) at = 60° ie cee 10.0 

Deutero-albumose ...). +. 2s pas eleceeeeeun tee oe 
Cane sugar’. os oo... ls an hw Oe ae ae co 
rai isco ati oe yp ain hs aoe ees (ore) 
Stannie acid sol (old) . .cc.0.c6 seme fore) 


W. Menz *** found that dilution increased the protective action — 
of gelatin sols, which he believed to be due to the gelatin 
amicrons, a view confirmed by the ultramicroscopic studies of 
Elliott and Sheppard.??” 

C. A. Smith *? suggests a polariscopic method for detecting 
the presence of gelatin. He says: “A progressive increase in 
levoration (or mutarotation) obtaimed from a solution cooled 
quickly 35° C., accompanied by the production of a jelly after 
a change of approximately 4.7° V., is very positive proof of the 
presence of gelatin in any solution concentrated enough to jelly.” 

A. D. Little ** has found the following qualitative tests satis- 
factory: 

To a water solution of the material to be tested add a slight 
excess of acetic acid, cool thoroughly and then add a concentrated 
solution of tannic acid containing about 10 per cent. of common 
salt. The tannic acid-salt solution should be freshly filtered 
before use. In the presence of glue this gives a grayish yellow 
flocculent precipitate. It is to be remembered, of course, that 


31a Z, physik. Chem. 68, 129 (1909). 
sib J, Ind. Eng. Ohem. 18, 609 (1920). 
32 J. Ind. Chem. Eng. 12, 878 (1920). 
33 Private communication. 


CHEMICAL EXAMINATION OF GLUE AND GELATIN 143 


casein and certain other soluble protein substances may give a 
similar reaction. 

Glue in Paper. Boil several grams of the paper with water 
until the volume of the solution is only a few cc. Filter and cool 
thoroughly; add an equal volume of a cold 10 per cent. salt 
solution nearly saturated with tannic acid and freshly filtered. 
A light grayish yellow flocculent precipitate indicates glue (or 
casein). In case the paper contains starch and gives a precipi- 
tate by the above test, the test should be repeated after first 
hydrolyzing the starch in the water extract of the paper by 
means of hydrochloric acid. This is accomplished by adding 
sufficient HCl to give a 2 per cent. solution and adjusting on 
the steam bath until all the starch is converted to dextrose, or in 
other words, until a drop of the solution, when added to 5 cc. 
of very dilute iodine solution, gives no blue color. 

It is important, in applying these qualitative tests, that the 
solutions be cold. 

There are here given in condensed form essentially the methods 
of the Association of Official Agricultural Chemists for deter- 
mining metals, etc., in gelatin.** 

Ash.—Ash at low redness preferably in a muffle until the ash 
is white or grayish white. (Gelatin intumesces violently, and 
due care must be used.) 

Total Phosphorus——Treat the ash obtained as above with 
2-3 cc. of nitric acid (sp. gr. 1.42) and evaporate on the steam 
bath. Repeat the nitric acid treatment and take up in hot 
water containing a few drops of nitric acid. Precipitate and 
weigh the phosphoric acid as magnesium pyrophosphate, accord- 
ing to usual analytical methods. 

Nitrogen—Determine by the Kjeldahl method, or by the 
modifications of this method by Gunning and Arnold. 

Arsenic.—Heat 20 grams of gelatin with 75 cc. of arsenic-free 
hydrochloric acid, 1-3, in a covered vessel until all insoluble 
matter has flocculated and the gelatin dissolved. Add an excess 
of bromine water (about 20 cc.), neutralize with ammonium 
hydroxide; add either about 14 cc. of 85 per cent. phosphoric 
acid or 2 grams of sodium phosphate (Na,HPO,.12H,O), or 
34 Wor complete details see ‘“‘Assoc. Official Agricult. Chem. Methods,” 1920. I 


must thank C. R. Smith of the U. S. Dept. of Agriculture, Bureau of 
Chemistry, for information supplied. 


144 | GLUE AND GELATIN 


crystallized sodium ammonium phosphate and allow to cool. 
Precipitate the arsenic along with the phosphoric acid by an 
excess of magnesia mixture. The phosphoric acid or compound 
used should require about 20-25 cc. of the usual magnesia mix- 
ture for precipitation. After standing about an hour, wash the 
precipitate several times with dilute ammonium hydroxide, drain 
well and dissolve in dilute hydrochloric acid, 1 to 3, to 50 ce. 
volume, in a graduated flask. Take a 25 cc. aliquot and deter- 
mine the arsenic as directed below. Run a blank determination 
with the sample. Arsenic impurities, if present, are usually 
found in the phosphate added. 

The apparatus for determining arsenic consists of a 2-ounce 
wide-mouth bottle, closed with a thoroughly cleaned perforated 
rubber stopper in which is inserted a glass tube 1 cm. in diameter 
and 6 cm. long. A second tube like the first is connected with 
it by a clean rubber stopper, and on the top of this second tube 
is mounted in like manner a third narrower tube 3 mm. in 
diameter and 12 cm. long. Into the several tubes introduce the 
following: 

Furst (lowest) tube—A rolled piece of heavy filter paper, size 
about 4.5x 16 cm., which has been soaked in 20 per cent. lead 
acetate solution, and dried. 

Second tube.—Pack loosely with absorbent cotton, soaked in 
5 per cent. lead acetate solution and squeezed to remove any 
excess. Since the test involves comparison of stains, all tubes 
used should have cotton of uniform moisture content. 

Third (top or narrow) tube.—A strip exactly 2.5 mm. wide and 
about 12 cm. long of heavy drafting paper (similar to What- 
man’s cold pressed) which has been soaked for one hour in a 
5 per cent. solution of mercuric bromide in 95 per cent. alcohol, 
squeezed free from excess solution of mercuric bromide and 
dried on a glass rod. Both ends of the strip should be trimmed 
off before using. 

Place 25 cc. of the gelatin solution prepared as above described 
in the 2-ounce bottle and add 20 ce. of dilute arsenic-free hydro- 
chloric (1-3) acid. Warm to 90° C., add 3 drops of stannous 
chloride solution (40 grams stannous chloride made up to 100 cc. 
with concentrated hydrochloric acid). Heat for 10 minutes and 
then cool the bottle and its contents in a pan containing water 
and ice, to a temperature of about 10° C. 


CHEMICAL EXAMINATION OF GLUE AND GELATIN 145 


When cold, add about 15 grams of arsenic-free zinc in sticks 
about 1 cm. long, and immediately connect up the chain of tubes 
to the bottle. | 

Keep the bottle in the iced bath for 15 minutes to retard the 
evolution of gas, and then remove it and allow the action to 
proceed for one hour more. Remove the sensitized mercuric 
bromide paper, and compare the stain with similar ones pro- 
duced under identically the same conditions with known amounts 
of arsenic. 

A-standard arsenic solution is made by dissolving one gram 
of arsenious oxide in 25 cc. of 20 per cent. sodium hydroxide 
solution, neutralizing with dilute sulphuric acid, adding 10 ce. 
of concentrated sulphuric acid, and making up to one liter with 
recently boiled distilled water. One cc. of this solution contains 
1 mg. of arsenious oxide (As,O,). It may be diluted as desired; 
e.g., if 20 cc. be made up to 1 liter, and 50 cc. of this dilute 
solution be further diluted to 1 liter, there results a solution 
containing 0.001 mg. of arsenic oxide (As,O,) per cc. Such 
dilute solutions must be freshly prepared just before using. 

Copper.—Hydrolize 50 grams of gelatin with 150 cc. of dilute 
hydrochloric acid, 1 to 3, as directed under arsenic, heating about 
2 hours on the steam bath. To facilitate filtration and separa- 
tion from zinc and iron later, use the phosphoric acid or com- 
pound and magnesia mixture as before. Precipitate with hydro- 
gen sulphide in a slightly ammoniacal solution. Allow the pre- 
cipitate to settle, filter and wash with 5 per cent. ammonium 
chloride solution saturated with hydrogen sulphide. Dissolve off 
the zinc and iron sulphides, magnesium phosphate, etc., in 75 cc. 
of dilute hydrochloric acid (4 per cent. HCl) saturated with 
hydrogen sulphide. Digest both filter and copper sulphide with 
4 ec. of concentrated sulphuric acid and sufficient nitric acid 
until the residue is perfectly colorless and fuming freely. Take 
up with water, add 5 cc. bromine water, boil off the bromine, and 
determine copper by titrating with 0.01 N sodium thiosulphate. 

Lead.—lIf lead is present, it is shown as the sulfate mixed 
with some silica when the sulphuric acid residue is diluted with 
water in the above determination. Add an equal volume of 
alcohol and allow to stand several hours. Filter and wash with 
dilute alcohol. Evaporate the filtrate to remove alcohol and 
determine copper as directed above. 


146 GLUE AND GELATIN 


Dissolve the lead sulphate from the filter with 10 cc. of hot 
50 per cent. ammonium acetate solution alternated with hot 
water until the filtrate measures about 75 cc. Add potassium 
dichromate solution to precipitate the lead as chromate, filter on 
a dry Gooch at 125-150° C. and weigh. Calculate to pases 
lead using the factor 0.641. 

Zinc.—Determine the zinc in the filtrate from the copper 
determination as follows: 

Boil the filtrate containing the zinc, to expel hydrogen sul- 
phide, add a drop of methyl orange indicator and 5 grams of 
ammonium chloride and make alkaline with ammonium hy- 
droxide. Add dilute hydrochloric acid drop by drop, until the 
reaction is faintly acid, then add 10-15 cc. of 50 per cent. sodium 
or ammonium acetate solution and pass in hydrogen sulphide 
for a few minutes until the precipitation is complete. Allow the 
precipitate to settle, filter until clear, and wash the precipitate 
twice with hydrogen sulphide water. Dissolve the precipitate 
on the filter with a little hydrochloric acid (1-3), wash the filter 
with water, boil the filtrate and washings to expel hydrogen 
sulphide, cool and add a distinct excess of bromine water. Then 
add 5 grams of ammonium chloride and ammonium hydroxide 
until the color, caused by free bromine, disappears. Add hydro- 
chloric acid (1-3), drop by drop, until the bromine color reap- — 
pears, then add 10-15 cc. of sodium or ammonium acetate solu- 
tion (50 per cent. by weight) and 0.5 ce. of ferric chloride solu- 
tion (10 per cent.) or enough to precipitate all the phosphates. 
Boil until all the iron is precipitated. 

Filter while hot and wash the precipitate with water contain- 
ing a little sodium acetate. Pass hydrogen sulphide into the 
combined filtrate and washings until all the zine sulphide, which 
should be pure white, is precipitated; filter upon a tared Gooch 
crucible, and wash with hydrogen sulphide water containing 
a little ammonium nitrate. Dry the crucible and its contents 
in an oven, ignite at a bright red heat, cool and weigh as zinc 
oxide. ‘Calculate the weight of metallic zinc using the factor 
0.8034. 

The zinc may also We determined direct from the filtrate from 
the copper determination, by proceeding as directed in the second 
paragraph of the alternate method for copper and zinc, given 
below, beginning “Boil the filtrate,” etc. 


CHEMICAL EXAMINATION OF GLUE AND GELATIN 147 


Alternate Method for Copper and Zinc—Hydrolize 20-50 
grams of gelatin with 100 cc. of dilute hydrochloric acid, 1 to 3, 
for two hours on the steam bath. Add 5 mg. of iron from 5 cc. 
of a standard solution of ferrous sulphate (4.9 grams of ferrous 
sulphate to a liter containing 10 cc. of sulphuric acid). Make 
solution faintly ammoniacal and saturate with hydrogen sulphide. 
Filter the sulphides and wash 2 or 3 times with a very dilute solu- 
tion of colorless ammonium sulphide (saturate a solution of 1 cc. 
of concentrated ammonium hydroxide in 200 cc. of water). Dis- 
solve the sulphides in 20 cc. of hot, dilute nitric acid, 1 to 1, 
and wash the filter and insoluble matter with water. Add 10 ce. 
of dilute sulphuric acid, 1 to 3, and evaporate all the nitric acid. 
Cool and add 40 cc. of water. When the soluble salts are in solu- 
tion filter off silica, washing filter thoroughly with water. Satu- 
rate the filtrate with hydrogen sulphide. Heat the solution 5 
minutes on the steam bath. Filter the copper sulphide on a care- 
fully prepared Gooch crucible and wash with hydrogen sulphide 
water. Dry and ignite to copper oxide. 

Boil the filtrate to expel all hydrogen anibiiae Make the 
solution strongly ammoniacal and then acidify with 15 cc. of 
50 per cent. formic acid. Filter off any insoluble matter such 
as alumina, etc., while hot, then pass in a rapid stream of hydro- 
gen sulphide for 10 minutes. Warm solution 15 minutes on the 
steam bath, remove and allow to stand for 30 minutes before 
filtration. Filter the zinc sulphide on a carefully prepared Gooch 
erucible with a very gentle suction, washing with 2 per cent. 
ammonium thiocyanate. Dry and ignite at the highest tem- 
perature of a Bunsen burner. Cool and weigh the zinc oxide. 

Polariscopic Constants——Prepare a concentration of 3 grams 
per 100 c. by soaking 3 grams of the sample in 40-50 cc. of 
cold water for about 15 minutes, heating to complete solution at 
. about 50° C. and making to volume at 35° C. Polarize at 
35° C. in a 2 dm. tube using the Ventszke scale. 

Cool a portion of the gelatin solution rapidly to 10-15° C. 
and pour into cold dry 1 dm. tubes before jelly has time to form. 
Place the tube in a constant temperature bath at 15° C., and 
polarize after 18 hours to obtain equilibrium rotation at 15° C. 
Double the reading to place it on a basis of 2 dm. tube. 

In order to polarize cloudy samples, digest the original 100 ce. 
in a stoppered flask with roughly 10 cc. of lightly powdered mag- 


148 - GLUE AND GELATIN 


nesium carbonate for at least one hour at 35-40° C. and filter 
until clear through a folded filter, avoiding unnecessary evapo- 
ration. (This produces considerable hydrolysis J. A.) 

The increase in levorotation (mutarotation) between 35° and 
15° is an index of the jelly strength developed. 

Sulphur Diox.de—Proceed as in the distillation method de- 
scribed below, or by diffusion method as follows: Cool, in ice 
water, a vessel containing 100-150 cc. of water, 5 cc. of dilute. 
hydrochloric acid, 1 to 3, 10 grams of sodium chloride and some 
filtered starch solution. Add a few drops of 0.01N iodine until 
a blue color is produced. Pour this mixture on 5 grams of 
powdered gelatin sample contained in a stoppered flask, replac- 
ing in the ice water. After remaining for 2 minutes with occa- 
sional mixing, add 0.01N iodine until the blue color is restored. 
Replace in the ice water for one minute, remove and titrate to 
the reappearance of the color. Repeat these operations until 
the color persists for one minute. 

One cc. of 0.01N iodine is equivalent to 0.32 miligram of 
sulphur dioxide. 

Distillation Method.—Dissolve 20 grams of gelatin in 300 DG. 
of recently boiled water, add 5 cc. of a 20 per cent. glacial phos- 
phoric acid solution, and distil in a current of carbon dioxide 
until 150 cc. have passed over. (The current of carbon dioxide 
may be replaced, without material error, by using double the 
quantity of phosphoric acid and immediately before connecting 
up the condenser, dropping in a lump of sodium bicarbonate 
weighing less than one gram.) 

The distillate should be collected in 100 cc. of nearly saturated 
bromine water, the condenser end dipping below its surface. 
On completion of the distillation boil off the excess bromine, 
dilute the solution to about 250 cc., add 5 cc. of hydrochloric 
acid (1 to 3), heat to boiling, and precipitate the sulphurie acid 
with 10 per cent. barium chloride solution. Boil for a few 
minutes longer, allow to stand overnight in a warm place, filter 
on a tared Gooch crucible, wash with hot water, ignite at a 
dull red heat, and weigh as barium sulphate. 

Irving Hochstadter *° advises the use of 27.3 grams of gelatin. 
The number of milligrams of BaSO, then indicate directly the 
number of parts per million of SO,. The CO, should be washed 


35> Private communication. 


CHEMICAL EXAMINATION OF GLUE AND GELATIN 149 


through CuSO, solution to eliminate sulphides. He distils over 
200 to 250 cc. and catches it in 25 to 50 cc. x iodine solution. 


The former official method was to collect the distillate in 
standardized iodine solution, and then determine the excess of 
iodine by titration with standardized sodium thiosulphate. This 
method was severely criticized *®* and was abandoned. Parts of 
animals slaughtered under Government supervision as well as 
gelatin made therefrom and gelatin containing no added sulphur 
dioxide or sulphites, all showed apparent sulphur dioxide. C. 
Mentzel ** found in pure chopped meat from 0.0014 to 0.0021 
_ per cent. of apparent sulphur dioxide equivalent to from 0.0054 
to 0.0084 per cent. of sodium sulphite. When onions were added 
to the chopped meat, the percentage of apparent SO, was largely 
increased, probably owing to the presence of allyl sulphide. 

The substance responsible for apparent SO, in meat, gelatin, 
etc., is possibly the sulphur-containing protein cystine. It 
should be noted that, while meat contains about 70 per cent. of 
water (besides much fat), gelatin contains only about 15 per 
cent. of water, and in it all water-soluble substances would natu- 
rally be concentrated. Meat may absorb SO, from the atmos- 
phere,*® and Poetschke (loc. cit.) found the same condition 
with gelatin. Another possible source of error was pointed out 
by Baythein and Bohrisch,*® who observed that the limestone 
or marble used to generate CO, sometimes contains sulphides, 
and the gas should be washed with copper sulphate. 

From the results of the analysis of over 1,000 samples 
Poetschke reports as follows: *° 

SO, Content or SAMPLE 


Less than From 100 to Over 
100 parts 500 parts 500 parts 

Year per millton per million per million 
Me ile eke sacks ss 19.65 44.87 35.48 
ROMs as is bone t5 40 60.62 - 18.61 20.70 
i Oe 66.64 18.92 14.44 
RR dig ors vc cles i0 8 66.47 23.92 9.60 
COON a re 87.90 9.16 2.94 
BER as cs'tsseas s 48.27 42.65 9.08 


36 J, Alexander, J. Am. Chem. Soc. 29, 783 (1907) ; E. Gudeman, J. Ind. Eng. 
Chem., Vol. I, No. 2 (1909) ; P. Poetschke, J. Ind. Eng. Chem., Vol. 5, No. 12, 
(1913). 

31 Zeit. fiir Untersuch. Nahrungs und Genuss.. 11, 3820 (1906), 

88 A, Kickton, Zeit. fiir Untersuch Nahrungs Genuss. 11, 324 (1906). 

39 Zeit. fiir Untersuch Nahr. und Genuss. 5, 401 (1902). 

40'The figures given are the percentage of the total number of samples. 


150 GLUE AND GELATIN 


Poetschke also found free chlorine in the CO, which would 
cause error by oxidizing SO,; it is also removed by washing 
through copper sulphate: He prefers the use of iodine instead 
of bromine in the gravimetric method for determining SO,, and 
finds Gudeman’s steam distillation method shows no advantage. 
Some samples of gelatin contained hydrogen peroxide evidently 
added to oxidize the SO,.* 

As a result of these analytical uncertainties, traces of sulphur 
dioxide are disregarded in practically all jurisdictions. The fig- 
ures of Poetschke, including foreign as well as domestic gelatins, 
show a very creditable and successful effort on the part of gelatin 
manufacturers to comply with the official regulations. 

“Irving Hochstadter (U. S. Pat. 1,412,523, dated April 11, 1922), has pro- 
tected the process of bleaching foods (including gelatin) with SO., and then 


oxidizing any traces of this gas or of sulphites that may remain into sulphates 
by means of H2O, or other peroxide. ‘ 


Chapter 11. 
Technology of Glue and Gelatin. 


The technical operations in the manufacture of glue and 
gelatin may be grouped under the following heads: 


1. Stock or raw material and its treatment prior to boiling. 
2. “Boiling” or cooking, that is preparing a solution of glue or 
gelatin from the treated stock by the action of hot water. 

3. Clarifying, bleaching, filtering, evaporating or otherwise 
treating the dilute glue liquor. 

Chilling the glue liquor to a jelly. 

Cutting, spreading and drying the jelly. 

Packing, breaking, or grinding the dried glue. 

Testing, grading, and selecting or blending the finished 
product. 


sb a 


The treatment of glue stock varies considerably, but the opera- 
tions of glue manufacture subsequent to boiling, are along the 
same general lines in most factories, although there are differ- 
ences in apparatus and in chemical and mechanical treatment. 

Special machinery may effect the consolidation of several, or 
the elimination of one or more of the above operations. ‘Thus, 
if the glue is dried on a heated rotating drum, the operations of 
chilling, cutting and spreading are eliminated, the glue liquor 
being fed directly to the roll or drum. G.,. Illert? describes a 
complete plant (which he claims can be operated by three men), 
in which a concentrated glue-foam is fed automatically to a 


drying roll, the dry glue passing directly thence to the packing 
apparatus. 


Glue Stock. 


The most motley array of raw materials find their way to the 
glue factory. They include slaughter-house refuse such as heads, 
feet and bones from the canning department; butchers’ refuse; 


1“Die neuzeitliche Hinrichtung und der Betrieb einer Lederleimfabrik,’’ Chem. 
App. 8, 78 (1921). : 


151 


152 GLUE AND GELATIN 


dried bones (junk bone) which come largely from South America 
or India; bones from garbage; clean bone trimmings from button 
and handle manufacturers; horn pith (the cornellion or interior 
bony support of the horn); trimmings of hides and skins such 
as raw hide, calves’ pates, tannery trimmings and fleshings; 
shreds of rabbit, hare, nutria and other skins from the hatters’ 
fur cutting industry; sinews and pizzles. 

Glue stocks (and the glues which they yield) may be divided 
into four groups. 


1. Bone stock. Besides the kinds above mentioned, this in- 
cludes: ossein, which is degreased granulated bone from 
which the soluble lime salts have been leached by acid; 
“spectacles” (knochen-brillen, dentelles), a variety of ~ 
ossein showing the round holes from which buttons have 
been cut; acidulated horn pith, a variety of ossein show- 
ing the original shape of the horn. Acidulated bone (ossein 
and horn pith) yield very high-grade glues and gelatins, 
fully equal to those produced from the finest hide stock. 

2. Hide stock. This includes such curious things as old Turk- 
ish raw hide moccasins, lips and ears, hide bale wrappings, 
and discarded loom-pickers worn out from incessant impact 
of the shuttle. Drved-hide stock may have been limed be- 
fore drying; thus guaras negras is dried unlimed, while 
guaras blancas is dried limed hide stock from South Amer- 
ica. Wet or green hide stock is usually salted or limed to 
preserve it, but wet limed stock will not stand long trans- 
portation unless the temperature is low. 

3. Sinew stock. Sinews are imported in dried state, but local 
sinew stock is usually shipped in green salted condition. 
With the imported dried sinew stock are often included 
dried bull’s pizzles. 

4. Tanned stock. In recent years processes have been perfected 
for making glue from leather waste. 


Treatment of Glue Stock. 


Before describing in detail the steps in the treatment of glue 
stock prior to boiling, it should be borne in mind that with some 
stocks the complete. cycle of operations is unnecessary, while 
with other stocks special treatment is required. 


TECHNOLOGY OF GLUE AND GELATIN 153 


Hide and acid leached bone stocks are limed, but rabbit skin 
stock is merely soaked. Many manufacturers cut or shred their 
hide stock so that it may be limed and extracted more readily. 
Sinews and pizzles are limed and treated like hide stock. 

The treatment of bones varies considerably, depending on the 
facilities of the plant and the products desired. The finest bone 
glues and gelatins are made by first converting the bone into 
ossein, which is accomplished by an acid leaching process (see 
p. 156). Many packers and slaughter houses aim to secure mainly 
“steamed bone,” and submit the fresh bone to one or two extrac- 
tions in a pressure tank whereby most of the grease and a large 
part of the glue are incidentally obtained. Plants operating on 
dried or junk bone usually granulate the bone and recover the 
remaining grease with volatile solvents, after which the glue is 
extracted. The poorest bone glues are usually made by the 
unskillful “boiling” of uncleaned and frequently unwashed garb- 
age or slaughter-house bone. Glues made from bones that have 
not been degreased, usually contain considerable grease. 

In general all salted stock is washed free from salt, limed 
stock, if old, is washed to get rid of old carbonated lime, and 
dried stock is soaked to soften and swell it. The glue manu- 
facturer must be on the alert to discover adulterations—thus a 
precipitate of barium sulphate is sometimes formed on hide stock 
to add to the weight. Even preservatives and disinfectants 
legitimately used may cause trouble; for example, arsenic is 
often used in curing hides, and the customs regulations of some 
countries require disinfection against anthrax, for which purpose 
bichloride of mercury, formaldehyde or sulphur dioxide may be 
officially prescribed. 

The following diagrammatic table shows the usual schemes of 
handling various stocks: 


Bone Stock. 


a. Wash and boil. 
Green, fresh or packer bone |b. Wash, grind, degrease and 


Refuse, town, or garbage bone boil. 

Dried or junk bone c. Wash, grind, degrease, acidu- 

Steamed bone late, lime, wash, neutralize 
and boil. 

Acidulated bone (ossein) Soak, lime, wash, neutralize and 


Acidulated horn pith boil. 


154 GLUE AND GELATIN 
Hides and Sinew Stock. 


Green or fresh trimmings or 
fleshings, also the same stock | Wash, lime, wash, neutralize and 
partly limed boil. 

Salted hide or sinews 


Dried hide pieces or ned oem wash, lime, wash, neutral- 


(whether previously limed or se nny Nea 


not) 


Leather or Tanned Stock. Soak, detan, lime, neutralize and boil. 


Modern practice is to cut up hide and leather stock to render 
subsequent operations more rapid. 

Water Supply and Sewage Disposal in the Glue and Gelatin 
Factory. Whatever salts or other non-volatile impurities exist 
in the water with which the glue solution is made, will be found 
in more concentrated state in the dried product. Where such 
impurities accumulate in a steam boiler, they may be largely 
removed by blowing down the boiler from time to time; but 
there is no practical way of removing them from the glue solu- 
tion. | 

The importance of pure water in a gelatin factory, is, there- 
fore, obvious, and for especially pure products filtration or even 
distillation may be advisable. i 

An abundant supply of water is essential, as well as adequate 
provision for the disposal of the large volume of wash-water 
which may cause difficulty owing to the fact that at times it is - 
liable to putrefy. Putrid glue has a peculiarly nauseating odor, 
and the sewage from a glue factory requires even more con- 
sideration than that from a tannery. 


Methods and Apparatus for Preparing Glue Stock. 


Bone Stock. To remove metallic and other impurities which 
would otherwise cause costly damage to machinery or contami- 
nate the glue or gelatin, the bones are first sorted and passed 
before a powerful electro-magnet which removes bits of iron 
concealed by fat or dirt. 

The bones then pass to the crushing or granulating machines. 
In Europe heavy-toothed steel rolls driven by powerful spur gears 
are generally used for crushing, but high-speed percussion mills 


TECHNOLOGY OF GLUE AND GELATIN : 155 


may be used providing the bone dust is sifted out. Stamp mills 
are also used. ea ak 

Bones are often given a preliminary boiling to remove the 
major portion of the grease. 

For the extraction of grease from the crushed bone, petroleum, 
benzine or naphtha is generally employed, although coal tar 
benzene, carbon tetrachloride, or carbon bisulphide may be used. 
According to H. G. Bennett, the petroleum fraction boiling at 
about 212° F. is now used in most British factories. All of it 
must be volatile at 280° F., and the last traces are blown out 
of the bones with steam at 80 lbs. pressure. L. Thiele? reports 
the use for bones of a mixture of 20 parts benzol, 60 parts toluol, 
and 20 parts xylol, and gives distillation tables of this and of 
three satisfactory benzines. A suitable benzine, according to 
Thiele, should have a sp. gr. of 0.745 and boil at about 100° C.; 
99 per cent. should distil over at 180° C. and any balance between 
130° and 140° C. 

Carbon tetrachloride has a low boiling point, gives light- 
colored grease and above all is non-inflammable; but its cost is 
high, special tin-lined apparatus is necessary, and the loss in 
solvent value is high. The use of naphthalene as a solvent has 
been patented, but it is not used. 

There are many types of apparatus suitable for extraction, 
and since improvements appear from time to time, the makers 
of such machinery should be consulted for latest details.* De- 
scriptions of extraction apparatus are also to be found in most 
standard books on chemical engineering. | 

The general principle on which most extraction systems work, 
is to have the solvent percolate through a granulated bone con- 
tained in a closed tank, the fat-containing solvent being caught 
in a still set at a lower level. The solvent vapors from the still 
are condensed and the fat-free solvent is again sent through 
the bone. When extraction is complete, the condensing solvent 
is diverted to a storage tank, and the solvent remaining in the 
bone is driven out by steam, a special separator being used to 
part the solvent and the condensed steam. The apparatus is then 


2“Glue and Gelatine,”’ 2d ed., M. Jiinecke, Leipzig. 

3 Nothing is to be gained here by burdening the book and the reader with a 
diagram of some selected form of extraction apparatus and details regarding 
the operation of its valves, etc. This would simply serve to perpetuate types 
which may soon become obsolete because something better has been discovered. 


Loony GLUE AND GELATIN 


opened, the extracted bone removed, and the solvent-free fat 
run off. 

Before boiling, the degreased bones are freed from dirt and 
adhering meat by being rotated in a large perforated screen or 
drum called a rattler or cleaner, in which they are polished by 
auto-attrition. 

Thiele reports the following average yield from raw bones: 


(Crushes DODGE lee. born wen ten eee 50.3 to 59.5 per cent. 
Bonetadusticy Pye eo. cate SG at ataw 

Bone aber tay ser eet ido eee 

HGiS Volts ic eco ee ee oo ie 

Horie ids oh eee ae ees 0.01 “ 0.04 
Tennongs ate cok: donecte ees 0.19 "Fis 

SONS eis Meee sc eee ae ee ae 0.02 “ 0.10 


The degreased bone has from 5 to 6 per cent. of glue and about 
60 per cent. of calcium phosphate. 

The acidulation of the degreased bones is Hefei accomplished 
by dilute (about 8 per cent.) hydrochloric acid and takes place 
in large wooden vats, which may be subjected to intermittent 
rotation, or through which the acid solution is slowly circulated 
by pumps. The counter current system of circulation is used, 
and the time required for treatment varies with the nature and 
size of the bone, being from 8 to 10 days with occasional limits 
of 4 to 14 days, according to Bennett, and from 2 to 3 days, 
according to Thiele. Degreased bones are attacked more rapidly 
than steamed bone. According to Bogue ** American practice 
is to use from 2 to 5 per cent. hydrochloric acid, one pound of 
bone requiring roughly one pound of 22° Beaumé acid for com- 
plete extraction: 

In acidulation the main reactions are as follows: 

Ca,(PO,), + 4HCl = 2CaCl, + CaH,(PO,), (acid phosphate) 
-  Ca,(PO,), + 6HCl = 3CaCl, + 3H,PO, 
Ca,(PO,), + 4H,PO, = 3CaH,(PO,), 

The acid phosphate is then precipitated by carefully adding 

milk of lime: 
CaH,(PO,), + CaO = Ca,H,(PO,), + H,O 
H,(PO,),-++ 2CaO = Ca,H, (PO,), 4a 
(Secondary reactions occur here to some extent.) 
To avoid an excess of lime which would cause a reformation 


sa “The Chemistry and Technology of Gelatin and Glue,” McGraw-Hill Book 
Co., 1922. 


TECHNOLOGY OF GLUE AND GELATIN 157 


_of tricalcium phosphate, filtered samples of the acid liquor are 
tested from time to time with molybdic acid solution. When the 
acid phosphate and free phosphoric acid are both completely con- 
verted into Ca,H,(PO,)., a precipitate of ammonium phospho- 
molybdate no longer forms, and the addition of lime is stopped. 
Should an excess of lime be accidentally used a suitable quantity 
of acid liquor may be added to retrieve the error. The precipi- 
tate, Ca,H,(PO,)., is then washed free from calcium chloride in 
a filter press. Since the phosphate readily hydrolyzes, as little 
wash water as possible is used. It is known as “precipitated 
bone phosphate” and is largely used in the manufacture of bone 
china and fertilizers. The ‘acid phosphate” is used in making 
phosphate baking powder. 

The soft collagen, after washing and neutralization of the 
residual acid with lime water, may be made directly into glue 
or gelatin. If dried at a low temperature it yields commercial 
ossein. : 

Other acids than hydrochloric may be used to acidulate bone. 
Sulphurous acid is the one most employed, though phosphoric 
acid has been tried. Sulphuric acid is not suitable owing to the 
formation of insoluble calcium sulphate which blocks the process. 

In the process patented by Grillo and Schroeder bones are 
disintegrated by moist sulphurous acid gas or by liquid sul- 
phurous acid according to the equation: 


fae = SO, | H,O — Ca,H,(PO,), + CaS0,. 


Bones thus treated readily dissolve upon boiling or steaming, 
yielding a “mud” which forms a valuable fertilizer after the 
calcium sulphite is oxidized to sulphate. 

The Bergmann process for decalcifying bones is as follows: 
The degreased bones are placed in closed tanks and a solution 
of sulphurous acid is percolated through them, its strength being 
maintained by continuous additions of sulphurous acid gas. 
This leaves a thoroughly bleached ossein which is washed free 
of acid. ‘The leach liquor is heated in a lead lined tank, liber- 
ating free SO, for further use, and precipitating calcium phos- 
phate and calcium bisulphite. 

The bisulphite is decomposed by hydrochloric acid, freeing a 
further quantity of SO, for return to the process, so that all 
told only about 5 per cent. of SO, is lost. 


158 GLUE AND GELATIN 


Dentelles consists of ossein made from button makers’ bone 
refuse. It is often called “spectacles” because the pieces of bone 
contain round holes which recall the appearance of eye-glasses. 
Prepared horn pith is a variety of ossein made from the cornel- 
lion or interior osseous core of the horn. Since it does not come 
in contact with flesh, and has a porous structure that renders 
easy its extraction, it produces high-grade gelatin. The acidu- 
lated horn pith of commerce keeps its original shape. Pieces 
of the skull bones are often left on, and if not properly leached, 
constitute “dead bone,” which adds to the weight but reduces 
the percentage of gelatin yielded. The yield from ossein is said | 
to vary from 65 to 85 per cent. 


Hide and Sinew Stock. 


The liming of hide, sinew and ossein stock has for its object 
the thorough swelling or “plumping” of the stock and the elimi- 
nation of mucin. The lime pits are square wooden or cement 
tanks about 4 feet deep, sunk in the ground like those of a 
tannery. The soaked and washed stock is thrown into a satu- 
rated solution of lime contained in the pits, and the stock is 
occasionally stirred up or transferred from one pit to another, 
with the aid of a long-handled fork, the lime solution being 
agitated, renewed or strengthened as often as necessary. ‘Thick 
hide pieces often have to remain in the lime vats several months 
before they are properly limed; thin fleshings or skivings lime 
much quicker, and in general liming proceeds more slowly in 
winter. 

In some plants to save labor the stock is pumped in and sucked 
out of the vats by large centrifugal pumps similar to those used 
in dredging operations, or else is handled by bucket chains. 

To shorten the time of liming, the hide pieces may be cut up 
or shredded. Furthermore caustic soda is often added to the 
lime liquor to “sharpen” it and produce a quicker swelling—too 
much must, of course, be avoided. With gelatin stock sodium 
peroxide is often used to produce a bleaching action at the same 
time. 


Washers. , 
To swell up and soften dried hide or sinew stock or to free it _ 
from salt or lime, it is treated in mechanical washers. 


TECHNOLOGY OF GLUE AND GELATIN 159 


The type most popular in America is the “cone washer.” 
This consists of a heavy slatted, hollow cone about 5 feet long, 
that is rolled by a rotating arm around a shallow circular tank 
about 10 feet in diameter through which a current of water is 
passing. ‘The cone presses and kneads the stock and the agita- 
tion results in thorough washing by the water which enters at 
the center and runs off through perforated grids at the outside of 
the tank. 

In Europe smaller tanks with rotating paddles are in com- 
mon use. 

G. Illert * describes a series of washers having horizontal arms 
turning at 90-100 R.P.M., the hide stock being passed from one 
to the other by bucket elevators. A copious spray of water 
removes lime, etc., through the perforated sides and bottom and 
the washed stock is automatically fed to a press, where it is 
squeezed before dumping into the boiling tanks. A capacity of 
10 tons per hour is claimed. 

After washing limed stock, it is usual to add some hydro- 
chloric or sulphuric acid to the last wash water and let the stock 
soak in it so that any remaining lime may be neutralized. This 
is readily determined by cutting open a piece of swollen stock 
and testing with litmus or phenolphthalein. Alum is also fre- 
quently added to the last wash water. 

The remaining acid is then washed out and the stock is trans- 
ferred to the cookers. Sulphuric acid usually clouds the glue by 
forming a small quantity of calcium sulphate. Hydrochloric 
acid forms calcium chloride which keeps the glue clear but which 
lowers the jelly strength if much be present, for whatever is 
- left in the stock remains in the finished glue. In making photo- 
graphic gelatin it is* particularly objectionable to have any 
quantity of salts left; calcium chloride is, of course, hygro- 
scopic. 


Tanned Stock. 


H. R. Procter® suggested that chrome tanned hide may be 
stripped of chrome for glue manufacture by a solution of 
Rochelle salt or other salt of an hydroxy-acid. M. C. Lamb 


4Chem. App. 8, 78 (1921). 
5 Soc. Chem. Ind. Annual Rept. on Appl. Chem., 1916, p. 232, 
6 J. Soc. Chem. Ind. 38, 572A (1919). 


160 GLUE AND GELATIN 


leaches the cleaned disintegrated chrome leather for 48 hours 
in a 15-40 per cent. solution of organic acids containing two or 
more hydroxyl groups, oxalic acid being preferred. The chrome 
is precipitated as hydroxide from the extract, and the regen- 
erated hide, after washing in weak alkali, is limed as usual to 
make glue. The results are said to be very satisfactory, which 
can hardly be the case with the drastic process of A. Wolff,’ 
who dissolves chrome leather waste in at least its own weight 
of 5 per cent. sulphuric acid. After removing the separated fat, 
the chrome is precipitated as hydroxide by lime, the lime re- 
moved and the filtrate dried for glue. 

S. R. Trotman ® dechromed hide for glue making by oxidizing 
the chrome to sodium chromate with sodium peroxide. W. 
Prager ® converts the basic chrome salt into the normal soluble 
salt by a 2 per cent. solution of sodium bisulphite. 

Lime and other bases have also been used to de-tan chrome 
leather,’® but the acid methods seem to be preferable. 


Boiling Apparatus and Methods. 


In the extraction, cooking or “boiling” process, the prepared 
stock is subjected to the solvent action of hot water or steam, 
whereby the swollen collagen is changed into gelatin or glue. 
Hofmeister regarded the change in a definite hydrolysis pro- 
ceeding in two stages according to the equations: 


Cro2HsgNsi08 + H,0 = Cy o2H15:N310 95 
Collagen + water = Gelatin 
Cro2Hy51N3i025 ++ 2H,0 = C5sH5N1;02 ae Cur HroN,Or9 
Gelatin + Water = Semiglutin + Hemicollin 


Considering the fact that the final disintegration products of 
gelatin are amino-acids, and that progressive heating results in 
progressive degradation, it is obvious that the much used term 
“hydrolysis” simply conceals our real ignorance of what actually 
does occur. In fact, Emmett and Gies claim that the process 
is one of molecular rearrangement and no hydrolysis at all. But 
even this does not seem to coincide with the experimental facts. 


™J. Soc. Chem. Ind. 38, 331A (1919). 
8 J. Soc. Chem. Ind. 30, 1462 (1911). 
®J. Soc. Chem. Ind. 32, 501 (1918). 
10 See e.g. German Patent 202,510, 


TECHNOLOGY OF GLUE AND GELATIN 161 


With colloidal substances like gelatin, chemical changes are | 
so closely associated with physical changes, that-no line of sepa- 
ration can be drawn between the two. Indeed, the whole boil- 
ing process appears to be a gradual breaking up of colloidal 
adsorption complexes (or of large ‘molecules’ held together by 
residual molecular attractive forces), with accompanying changes 
in free surface and amount of water adsorbed. 

The higher the extraction temperature, and the longer the 
stock and liquor are exposed to it, the more rapidly these degen- 
erative changes proceed, and the lower in test and darker in 
_ color the glue will be. Therefore, it is desirable to extract the 
stock as quickly as possible, and at the same time to keep the 
extraction temperature low. Since these two factors oppose 
each other there is a wide range of possibilities. Usually with 
gelatin where color is important, the temperature is kept low, 
even though the extraction period is thereby lengthened. With 
glues, especially bone glues, higher temperatures are generally 
used. 

Since the stock dissolves quicker in pure water than in glue 
solution, attempts to produce too concentrated a lquor unduly 
protract the period of extraction. 

It is really a misnomer to call the extraction process ‘“‘boil- 
ing,” for the boiling temperature is seldom reached, except in 
_ pressure tanks, or when extracting residues. The temperatures 
generally used vary from about 70° to 90° C., depending upon 
the kind and condition of the stock. Too low a temperature, 
which would favor bacterial growth and decomposition, must 
be avoided. | 

Whereas slow circulation of the glue liquor (i.e. by rotary 
pumps) aids solution, unnecessary agitation is harmful as it 
tends to lower viscosity and jelly strength and cloud the liquor. 

Two types of kettles, or cookers, are used: (1) open tanks; 
(2) pressure tanks. 


Open Kettle or Tank. 


The open tank, used mainly for hide, sinew or ossein stock, 
usually consists of a rectangular or round wooden tub having 
a closed steam coil over which is placed a perforated or slatted 
false bottom of wood or iron, so as to leave a circulating space 


162 GLUE AND GELATIN 


between the two. Upon the false bottom is placed a layer of 
excelsior or straw often topped with a thin layer of hair, thus 
forming a rough strainer, upon which the stock is thrown until 
it reaches within a foot or so of the top of the tank. Sufficient 
pure water is then added (almost enough to cover the stock) 
and steam is turned into the coil until the desired temperature 
is reached; whereupon the steam is cut down to minimum 
necessary to maintain this temperature. As a rule the first 
“run” or “boiling” is made at about 70° C. (158°), and subse- 
quent runs are made at progressively increased temperatures. 

During boiling the stock is occasionally “opened up” by | 
stirring with a long pole, so as to permit a more perfect circula- 
tion of the liquor, or a circulating “chimney” is provided. From 
time to time a sample of the liquor is tested by a hydrometer 
or by chilling it in a cup, and when a sufficiently concentrated 
“soup” is obtained, it is run off, fresh water is added, and 
another “run” or boiling is made. A boiling usually takes from 
2 to 6 hours, depending on the nature of the stock and the tem- 
perature used. ‘The last run or wash water is extracted at boil- 
ing heat and is usually so weak that it is added to another kettle 
or to a stronger run, or else must be evaporated. 

The residual tankage is used for fertilizer. It may contain 
considerable grease or insoluble lime salts of fatty acids. In 
this event it is boiled with sulphuric acid to liberate the grease, 
which is then squeezed out in an hydraulic press. 

During boiling in open tanks, most of the grease contained 
in the stock rises to the surface and is skimmed off. With 
fleshing stock the yield of grease is heavy, often exceeding the 
yield of glue in the case of machine fleshings which take in 
relatively little of the hide or skin substance. . 


Pressure Tank. 


The pressure tank is largely used for extracting untreated or 
degreased bone stock, and acidulated bone. The tanks are ver- 
tical steel cylinders with convex ends, having large manholes 
at the top for filling, and at or near the bottom for discharging 
the spent bone. Hide glues made in pressure tanks are usually 
weak, and the tank is seldom used for hide stock. 

The bones may be boiled with water under a pressure of from 


TECHNOLOGY OF GLUE AND GELATIN 163 


10 to 20 lbs.; or hot water may be allowed to trickle in from 
the top while steam enters from the bottom (English process) ; 
or the condensation of the steam may supply the necessary 
water. ‘The successive runs of glue obtained are more concen- 
trated than those yielded by open tanks, though they do not 
have so strong a jelly. They are drawn off from time to time 
through a perforated false bottom. The most modern practice 
is to work pressure tanks in gangs upon the counter-current 
principle. Pressure tanks with circulating mechanism for 
handling ground bone have also been patented. 

If bones are intended for making bone black, they are either 
degreased by volatile solvents or else they are given one light 
cooking to remove most of the fat and but little of the glue. 
If too much of the nitrogenous matter is removed, the bone 
black will be of poor quality. 

After extraction of the glue, the bones are dried in a rotary 
drum dryer, ground in a high-speed rotary: mill, and sold for 
fertilizer. Thiele reports the following analyses of bones from 
which the glue has been extracted. 


Bones 
previous extracted 

with benzine Boiled bones Steamed bones 
Vo eS a 8.54 to 9.25 10.81 10.79 to 12.18 
Organic matter ........ 1/1074 19.53 25.97 22.48 “ 24.62 
Ca and Mg carbonates.. 7.50 “ 8.74 6.28 6.33 “ 6.89 
A IS Ses O02) "0.59 1.07 O86 25 
Ca poospnate ......... Ghigo 00410 53.15 5d04 Fo Bia 
LO os Trace 0.27 Trace 
(OS CONE 38sto. 178 2.45 168 to 1.74 
OST... ee 068 “ 1.05 1.91 164°“ 1.72 
Equivalent in glue..... Gios. C06 10.06 9.10 “ 9.56 


He also gives a detailed description of the operation of a 
diffusion battery of four pressure tanks in making bone glue 
(loc. cit., p. 50). He also describes (p. 98) a gelatin extractor 
with false bottom, in which the stock is treated with a spray 
of superheated water. He recommends glass enameled vessels 
for handling the gelatin liquors, which come off continuously 
and may be collected into one “run” or divided into various 
runs or fractions. 

The liquor resulting from the first extraction of the stock is 
known as the “first run,” that from the second extraction the 
“second run,” etc. The first run, having been subjected to less 


164 GLUE AND GELATIN 


heat for a shorter time naturally yields the strongest glue. The 
various runs may be kept separate (known as ‘‘successive glues’’) 
or they may be mixed together and dried as one batch. The 
cheaper bone glue liquors are often mixed with hide glue liquors, 
the resulting glue being known in Germany as “misch-leim.” 
In America such mixtures are usually made by mixing the sepa- 
rately granulated glues, although factories producing both hide 
and bone glues often mix the liquors. 


Clarification, Bleaching and Evaporation of Dilute Glue or 
Gelatin Liquors. 


While the removal of relatively coarse particles from glue 
liquors is readily effected by a strainer or filter press, the high 
protective or deflocculative action of glue makes difficult the 
separation of any finely subdivided or colloidal matter, which 
renders the glue turbid. The methods of clarification, apart 
from simple straining, fall into three groups: 


1. Mechamecal (settling or centrifugation). 

2. Adsorptive (filter-mass or bone black). 

3. Formation of precipitates (albumen, phosphoric acid, alum, 
etc.). 


About twenty years ago the writer was able to clarify gelatin 
liquor in an ordinary cream separator (DeLaval type) but the 
modern super-centrifuge (Sharples type) is, of course, much 
more efficient and is said to be much used in the United States 
for gelatin liquors. 

The use of paper or cellulose filter-mass, along the lines of 
modern brewery practice, and of bone black as per the methods 
used in sugar refining or glucose clarification, give excellent 
results; and they are widely used for gelatin. 

An old method of clearing liquors was to add blood or a 
solution of blood or egg albumen and then boil. The albumen 
in coagulating would carry down most fine turbidity; but the 
heating weakened the glue or gelatin, and gave a soapy smell 
to the gelatin. In the Grillo and Schroeder process the precipi- 
tate of calcium sulphite carries down most suspended matter, 


11 Several varieties of filters are on the market, details regarding which may 
be obtained from the makers. 


TECHNOLOGY OF GLUE AND GELATIN 165 


and the use of phosphoric acid is very common in clarifying 
gelatin liquors. If made from bone stock they may have sufi- 
cient lime to give the desired precipitate of calcium phosphate; 
otherwise milk of lime may be added as is done in the defeca- 
tion of sugar liquors. The glue liquor must be warm enough 
to let the precipitate settle, but the temperature must be kept 
as low as possible to avoid loss of strength. Phosphate of soda 
may be used, but has the disadvantage of leaving soluble sodium 
salts. Oxalic acid may. also be used, producing a precipitate of 
calcium oxalate. 

Alum or aluminum sulphate is the clarifying agent most used 
for glues. It seems to undergo hydrolysis in the presence of 
the colloid, and the nascent hydrate of alumina combines 
with the glue or the impurities giving a precipitate which 
can be settled or filtered out. Lambert?” gives the following 
details: 

“A bucketful of liquor, which should have a temperature of 
about 80° C., should be drawn from each vat, the necessary 
quantity of alum stirred in, and the contents thoroughly mixed 
in the mass, the heat at the same time being raised to 100° C. 
by means of a steam pipe. After boiling ten minutes the steam 
is turned off and the liquor allowed to settle, during which the 
heavier mineral and organic impurities fall to the bottom, while 
the lighter form a coagulated scum on the surface.” 

Such a procedure is obviously very injurious to the strength 
of the glue; in fact, as a general rule highly clarified glues are 
relatively weak, strength being sacrificed for appearance. 

Schwerin ** has patented a process for clarifying gelatin and 
glue liquors by electrodsmosis, but this method has not yet found 
general application. 

H. Fleck * describes a process for improving glue by precipi- 
tation with ammonium sulphate or sodium bisulphite, which 
removes some of the products of hydrolysis. He warned against 
the danger of boiling glue solutions. 

Of course, any grease rising to the surface of the glue liquors 
is skimmed off, though with cheap bone glues, when grease is 


122“Glue, Gelatine and Their Allied Products,’ London, 1905. 

23 “Kolloid Z. 20, 64 (1917), Ger. Pat. 293,188 (1918). 

144“The Manufacture of Chemical Products from Animal Offal,’’ Brunswick, 
1878. 


166 : GLUE AND GELATIN 


low in price, as much grease as possible is kept in the glue 
liquor and helps to render the glue free from foam _ besides 
increasing the yield. 

For the bleaching of glue, sulphurous acid or bisulphites are 
most commonly used because of their cheapness. The stock 
may be bleached before boiling, or SO, gas or its solution may 
be added to dilute or to concentrated liquors. Sodium hydro- 
sulphite and analogous compounds (zinc hydrosulphite, sodium 
formaldehyde sulphoxalate) have certain advantages, but are 
more expensive. 

For bleaching gelatin liquors, hydrogen peroxide may be used, 
but it is preferable to bleach the stock before boiling, in which 
case sodium peroxide or sulphurous acid are also of service. 
Peroxides also oxidize sulphites to sulphates. — 


Evaporation. 


Many first or second run gelatin and glue liquors, as they 
come from the boilers, will set firmly enough to be chilled, cut, 
and dried; but in most cases it is necessary to evaporate or 
concentrate the liquors. 

The old style steam worm rotating in a trough-like tank has 
been superseded by the modern double or triple effect evaporator. 
It is not proposed to burden the reader with a detailed descrip- 
tion of the various types of vacuum evaporators, which are being 
continually improved; for the apparatus used to-day may be 
superseded by a superior form soon after publication of this 
book, if not before. The apparatus is well known, is described 
in many elementary books, and the various manufacturers are at 
all times ready to give the latest details. 

The film evaporators (Yaryan, Kestner, Lillie, and Blair- 
Campbell) are in high favor, since in them the glue liquors 
are but a short time under the action of the heat, and this is 
conducive to maintenance of strength. 

The degree of evaporation depends upon: - 


1. The jelly strength of the original glue liquor; 

2. Its initial concentration; 

3. The temperature of the outside air. Weaker glues and 
warm weather naturally demand greater evaporation. 


TECHNOLOGY OF GLUE AND GELATIN 167 


Antiseptics. 


Among the antiseptics commonly used in glues are zinc sul- 
phate (which tends to precipitate odor-producing decomposition 
products), boracic acid, borax, sulphurous acid and bisulphites, 
and formaldehyde (about 1-10,000). Beta-naphthol may be 
used in the limes if desired. Phenol (carbolic acid) has also 
been used on stock and in liquors, but the odor is very objec- 
tionable. 


Chilling. 

In most factories the glue liquors, after evaporation and other 
treatment, are poured into small rectangular galvanized iron pans 
and allowed to gelatinize, preferably in a chill room. Some- 
times the pans are allowed to stand in cold water. Many 
European factories have casting tables, which form the glue 
directly into cakes. These tables have hollow tops through 
which is circulated cold water or brine; the upper surface (of 
metal or glass) is divided into square or oblong recesses which 
are filled with the glue liquor and which often have a trade mark 
etched in, so that it appears on the glue cake. 

Many devices have been patented for chilling concentrated 
glue liquor into a continuous sheet, which is automatically spread 
upon nets. None of these have found general widespread use, 
and several have been tried and abandoned. One of the early 
patents was that of Peter Cooper-Hewitt (U.S. P. No. 11,426, 
issued 1894). Some of the European gelatin factories are said 
to employ such apparatus. In the United States the apparatus 
of Maurice Kind (U. 8S. P. No. 1,046,307, issued 1912) is em- 
ployed by several factories.1° This machine has an endless 
belt which passes through a refrigerating tunnel, and on which 
is formed a continuous sheet of jelly of the desired thickness. 
The continuous sheet of chilled jelly coming from the belt is 
automatically cut into sheets of the required size, and these 
sheets are automatically spread upon drying nets of the usual 
kind. Fifteen minutes after leaving the evaporator, the first 
sheets are ready to enter the drying room. 

A single unit of the Kind machine is said to have a capacity 


15 See Arthur Lowenstein, Trans. Am. Inst. Chem. Eng. 10, 105 (1917). 


168 GLUE AND GELATIN 


of 4300 lbs. of dry glue per day of 20 hours with a 16 per cent. 
jelly spread 14” thick. The unit occupies a space of about 
85 feet by 10 feet, takes about 10 horse power, and requires 
about 10 to 12 tons of refrigeration. 

The advantages of such an automatic machine are obvious. 
Since gelatin solution is a particularly good culture medium, the 
more rapidly a glue jelly can be handled and dried, the less 
chance there is of contamination and bacterial decomposition. 
This applies especially to food gelatins which, because of the 
absence of antiseptics, are especially susceptible to the attack 
of bacteria. There is furthermore a big saving in labor, and 
more or less freedom from the effect of weather conditions. 

Glue and gelatin are frequently dried upon steam-heated rolls, 
which may rotate in a vacuum (Passburg system) or which may 
have a close-fitting hood forming a narrow channel through 
which a rapid blast of air carries off the evaporated moisture. 

Where vacuum or forced draft drying rolls are used, the chill- 
ing process is eliminated, for the concentrated glue liquor is fed 
to them direct. A recent German method (Ruf system) is to 
beat the thick glue liquor to a foam before feeding it to a 
steam-heated drum.® 


Cutting, Spreading and Drying. 


Where glue has been chilled on casting tables, the sheets of 
jelly are picked off by hand and spread upon the drying frames 
or nets. But where the glue has been chilled in pans or boxes, 
the large jelly blocks are removed (usually by dipping the pans 
for an instant in hot water) and cut into slices which are spread 
upon the nets. With large pans, the jelly blocks are first cut 
into several smaller blocks before slicing. | 

Any heavy impurities or particles of dirt-in the glue or gelatin 
liquor settle to the bottom of the jelly blocks, while grease and 
light particles float to the top. Therefore in most factories the 
blocks are sliced horizontally, and the ‘‘tops and bottoms” dried 
separately to make an inferior grade of goods, or else they are 
worked into another batch. If the jelly blocks are sliced ver- 
tically, the top and bottom impurities, if any, are distributed 
throughout the sheets. . 

16 See G. Illert, Chem. App. 8, 78 (1921). 


TECHNOLOGY OF GLUE AND GELATIN 169 


Most slicing contrivances have a grid of steel wires through 
which the jelly block is forced by a plunger; the distance be- 
tween the wires regulates the thickness of the slices. Or the 
blocks on a moving belt are carried against wires, some distance 
apart, and fixed at different heights. With hand cutters the 
jelly block is sunk below the surface of a table and raised inter- 
mittently by a ratchet;.between each successive elevation, a wire 
stretched on a bow-like handle is drawn through the block, 
which is thus cut into slices whose thickness depends on the 
number of teeth in ratchet. 

For very stiff jellies, knife cutters are sometimes used, but 
as a rule highly concentrated jellies show rough or wavy marks 
which detract from the appearance of the finished glue. 

The slices of jelly as spread on the nets vary from about 
3 to 10” in width, to about 6 to 12” in length. A certain auto- 
matic chilling and spreading machine which was operated for 
a time in one plant, spread a strip the size of a whole net. Rib- 
bon glue is cut in strips about 114 to 2” wide and about 8” long. 
Noodle glue is cut in strips having a cross section about 1%” 
square. Bazaar glue is similar to noodle glue, but of about 1” 
cross section. 

The thickness of the finished glue depends upon two factors: 
the thickness of the jelly slice, and the concentration of the 
jelly. The sheets of glue suffer more or less distortion on dry- 
ing, and usually show the marks of the nets on which they were 
dried. In Japan nets of bamboo are evidently used, as some 
Japanese glues show distinctly. Cast glues are usually highly 
concentrated, and the cakes tend to hold their shape well. The 
so-called Scotch glue is dark and comes in cakes about 10x 12” 
with a loop of string through one of the long ends. Thin flake 
glue is usually made from jellies strong enough to be dried with 
little or no evaporation. But thinness of flake is no criterion 
of quality, since concentrated jellies of weak glues are often cut 
very thin to simulate this supposed ear-mark of quality. Gela- 
tins are usually quite thin cut, especially those which are to be 
put up in paper packages holding about 1 pound. 

The drying nets upon which glue and gelatin are dried, usually 
consist of rectangular wooden frames about 3 x 5 feet, upon which 
are stretched pieces of galvanized iron wire netting with a mesh 
resembling chicken wire fencing. Some factories use cotton or 


170 GLUE AND GELATIN 


linen fish net, especially for gelatin, because if the zinc protec- 
tion cracks off the wire net, rust forms which injures the appear- 
ance of the product and may actually form insoluble brown 
flakes. 

The net frames may, of course, be made of metal, but each 
frame has a small block or leg in each corner so that when the 
nets are piled up into stacks there is a.space, usually about 1 
inch, between the frames for air circulation. 

The stacks of nets are mounted upon wheeled tracks or bogies, 
which generally run on tracks, and pass on into the dry room. 

In the old days the dry room was simply an upper floor or 
loft with louvred sides, and the drying was left to the vicissitudes 
of the wind and weather. Warm weather would melt the glue 
on the nets or foggy weather would pit or mold it. In all 
modern plants the glue is dried by passing a current of warm 
air over the stacks packed in narrow alleys or tunnels. : 

Some prefer the forced draft or blower type of fan, whereas 
others prefer the exhaust or ventilating type which sucks the 
air through the tunnels. Since the latter type fan has a tendency 
to churn if pulling against much resistance, the positive pressure 
fan is generally used. 

The air entering the alleys is heated by passing it over banks 
or stands of steam pipes, any of the approved systems being 
used. Exhaust steam is utilized where possible. In order to 
prevent the air short circuiting and thus passing through the 
alley without taking up its quota of moisture from the glue, 
it is, of course, necessary that the stacks of nets fill the alley 
as completely as possible, and that the top, bottom, and sides 
of the alley be air-tight. Windows are usually provided, through 
which the temperature at various parts of the alley may be read 
from the outside. 

The alley itself may be straight or U shaped. The glue enter- 
ing the end furthest removed from the source of heat, first comes 
in contact with moist air that has suffered a drop in temperature 
because it has already evaporated water from the forward stacks. 
The stacks move progressively toward the pipe coils, near which 
they get their final “baking” before being removed from the 
alley. | 

Considerable experience and care are required to get the best 
results from a drying alley. Improper adjustment of the tem- 


TECHNOLOGY OF GLUE AND GELATIN 171 


perature to the particular batch may cause the glue to melt 
and run through the nets or sink into the mesh, forming pendu- 
lous drops (“titted” glue), which are difficult to remove with- 
out damage to the nets. The formation of a very thin surface 
skin (skinning over) is a partial protection against these diffi- 
culties, but if the surface skin is too thick or formed too rapidly, 
it impedes further evaporation. The addition of a small quan- 
tity of formaldehyde to the concentrated glue liquor is said to 
facilitate the formation of a skin, besides acting as an antiseptic 
and giving a stronger jelly. Excessive drying must also be 
avoided, for besides diminishing the yield, it is apt to cause the 
glue to crack or check. | 

The time required to dry a batch of glue depends upon the 
concentration of the liquor, the thickness of the jelly slices, the 
strength or quality of the glue and the weather conditions. 
Good practice requires the drying to be completed as speedily 
as possible, usually within about 24 hours. Any plant where 
drying takes from 10 to 30 days, as claimed by E. Sauer,” must 
needlessly tie up an enormous amount of capital, product and 
apparatus. 

In England and on the Continent the prevailing practice is to 
pick the dried sheets or cakes off the nets and pack them into 
bags or other packages. In America the glue is simply dumped 
off by inverting the net, and the sheets are still further crushed 
by passing them through a rough breaker whence they issue 
in flakes (flake glue). A large percentage of glue in America 
is ground to about 8 to 16 mesh in high-speed percussion mills. 
At one time barrels were the leading packages here, but bags 
are coming into wider use, both for flake and ground glues. 

The different batches or boilings of glue and gelatin, which 
may amount to several hundred or even several thousand 
pounds, are as a rule kept separate for testing and grading. 
Where large lots of one grade are required, it is common 
practice to mix a number of boilings and then to test the 
mixture. 

According to Thiele the various brands of sheet gelatin, which 
are usually sold in 1-pound packages for food purposes, are 
selected as follows: 


1% Kolloid Z. 17, 180 (1915). 


172 GLUE AND GELATIN 


Average number of 


Run of Gelatin Brand or label ' sheets per lb. 
| -forileand 22) eee Non? plus ultfa:.<5 scene 285 
Dae nats Bas Peaiien hates Maas Oo Gold extra and Gold....... 227-200 
DO ALONG s Hele cutee eee Silvers se Ge a ee 180 
tania oni eink soe ee CODDELe a des ss eee eee 136 
Hielone ts). co. s ocd. eat Blacks. Pues edie. ean ee 90 


The finished glue should be stored in a place of mean humidity 
and temperature. Excessive moisture is apt to cause the glue 
to soften and even be decomposed by molds or bacteria, whereas 
excessive heat and dryness tends to make the glue crack and 
check. According to E. Sauer ?§ if sheets of glue that have been 
stored for a long time in a very dry room are transferred to a 
very damp room, many sheets, often with a loud report, may 
burst into small pieces. The unequal absorption of water, which 
naturally takes place at first on the surface, produces a tension 
which forcibly relieves itself by breaking the sheet of glue. 
Low grade, highly hydrolyzed glue are especially apt to one 
or crack. 


Blow-down Processes. | 

Since the cutting, spreading and drying operations are expen- 
sive, and both time and space consuming, attempts are being 
made to “spray-dry” the concentrated glue and gelatin liquors 
along the lines now successfully used in making milk powder. 
If the cost can be gotten down, such processes should have a big 
future, especially for gelatin. 


Percentage Yield of Glue Stock. 

Since glue stock is for the most part a highly variable com- 
modity, definite yields can hardly be predicted without examina- 
tion; but the following table, prepared by a factory manager, 
gives the results of his experience with several varieties of stock: 


Percentage Yields 


Kind of Stock Glue Grease 
Green bones ........ ek ok ticks ee eee 10-12 8-10 
Dry bones. 45 ue ee ee eee 14-16 5- 7 
Green calf)... 2230 ee eee ae 18-22 3- 5 
Green salted hide, 7720.5 i. ee 16-18 3— 5 
Dry hide’ .2..s4. 25s aes Ses a oe Ce ee a 
Salted sinews ........ RE Par eect 18-22 5 ee i) 
Dry, SINCWS 3.44 un < chine leet eee ee 40-50 — 
Dry fleshings ... c.04.< 7st: » «sles es Ge 10-12 
Wet. fleshings °. .i4.< 10 acer ein 8-12 12-15 
Horse chide *(wet) i. Shoes ee eee 10-15 — 
Sheep ‘skin, (wet). 0\.. citen eee 6-10 . 10-18 


% Kolloid Z. 17, 183 (1915). 


Chapter 12. 
The Testing and Grading of Glue and Gelatin. 


At the outset let it be emphasized that there is no single 
chemical or physical test which will satisfactorily gauge the 
value of a glue or gelatin for all purposes. Many authors have 
recommended individual tests and while these may have some 
value for special purposes, the wisest and safest way for the 
factory or sales manager is to run a series of connected tests 
which will grade the glue or gelatin against preceding lots of 
the same type, and thus render possible uniform deliveries to 
the consumer, whatever his business may be. 

This view was voiced by E. G. Clayton,’ who says: “In con- 
clusion the observations seem to show that, whilst it would be. 
rash to form a judgment on glue from a single test the evidence 
afforded by a number would be irresistible. Glue may be shown 
by certain tests to be suitable for one purpose, though less per- 
fectly adapted for another. ‘The expert’s wisest system appears 
to be, not to rely upon single short-cut tests of general quality 
but to employ a number of methods, including any having 
especial bearing on the present or prospective uses of the glue, 
and then to base his conclusions on a consideration of all the 
results together.” 

Before describing a connected series of general physical test 
methods which are, with more or less modification, used in glue 
‘testing laboratories, let us review briefly some of the methods 
that have been proposed for testing and grading glue. Secrecy 
has been traditional in the glue trade, with the result that treas- 
ured methods may be inferior to other methods known to all. 
The references given are to books and papers published in the 
usual scientific journals. An exhaustive review attempting to 
fix priorities is not attempted. Some partially completed meth- 
ods of the National Association of Glue and Gelatin Manu- 
facturers are given in an Appendix. 

1“The Technical Examination of Glue,” J. Soc. Chem. Ind. 21, 675 (1902). 

173 


174 GLUE AND GELATIN 


Jelly Strength. 


One of the first tests proposed was the consistency test of 
Lipowitz 2 which determines the relative capacity of the jelly 
for bearing a weight. ‘The instrument used ° consists of a saucer- 
shaped piece of tinned iron, 1 inch in diameter, having a thin 
metal rod soldered vertically to its concave side, the upper end 
of the rod being provided with a small metal funnel. The rod 
slips loosely in a perforated metal strip which supports the 
apparatus. The saucer is placed with its convexity next to the 
jelly, and shot gradually poured into the funnel until the saucer 
breaks through. The total weight required to rupture the jelly 
indicates the jelly strength. Of course, the determination must 
be made at a definite temperature, and with a definite concen- 
tration of glue. A thick top “skin” must be avoided. 

More recently R. Kissling? stressed the importance of jelly 
strength test, which is often called Kissling’s test. J. Fels® 
describes it as “correct as a comparative method,” and more 
recently R. H. Bogue® has found that this test alone would 
correctly gauge about 75 per cent. of all glues. It is also known 
as the “shot test,” the “jelly test,” and the “finger test,” the 
last from the fact that the relative jelly strength may be easily 
fixed by pressing the jellies with the finger tips. F. Davidowsky ? 
describes a jelly tester used by the factory at Hamborn. It 
consists ® of a hemispherical weight, bearing a vertical scale, 
which slides in a guide cylinder that rests on the jelly with a 
broad flat rim, and bears also an adjustable pointer to fix the 
zero point. According to K. Kieser® Davidowsky’s method is 
largely used in Germany to determine jelly strength. ‘This cor- 
responds to our “shot test” in all practical particulars. 

Many mechanical contrivances have been suggested to fix the 
jelly strength of glue. Some depend upon measuring the depres- 
sion produced by placing a definite weight on the jelly, without 


2“Neue Chem.-tech. Untersuchungen,” Berlin, 1861, pp. 37-42. 

3’ “Allen’s Comm. Organic Analysis,” 4th ed., Vol. 8, 607. 

4*Chem. Z. 17, T26 (1898) ; ibid., 22, 450 (1898). 

5 Chem. Z. 56 and 70 (1897). 

®& Chem. Met. Eng., July, 1920. 

™“Glue and Gelatin Manufacture,” 5th ed., p. 26. 

8’ This instrument is practically the same as that described in Technical Note 
No. F-32 of the Forest Products Laboratory, Madison, Wisconsin, who will 
supply working drawings on request. 

® Kolloid Z. 28, 186. 


‘TESTING AND GRADING 175 


breaking through. The jelly tester of E. S. Smith 2° consists of 
a pressure chamber whose bottom contains a thin elastic rubber 
diaphragm; attached is a rubber air bulb to produce pressure 
and a manometer to measure the pressure produced, that re- 
quired to produce a certain depression being taken as Jelly 
strength. 

EK. T. Oakes and C. E. Davis! described the jelly tester of 
A. Schweitzer, which consists of a balance, one arm of which 
carries a plunger below and a beaker above, so that on adding 
water to the beaker a definite compression of the jelly beneath 
the plunger may be produced. 

W. H. Low” has proposed a modification of Smith’s instru- 
ment. The Forest Products Laboratory 1? describes the con- 
struction and operation of a tester with a constant weight 
plunger, whose depression in the jelly is a measure of the jelly 
strength. 

All these methods ieolve re ae the jelly strength through 
the skin of uncertain strength and thickness which forms on the 
upper surface of the jelly. J. Alexander ** devised a jelly tester 
which avoids this disturbing factor by casting the jelly into 
truncated conical blocks in brass cups of definite size, and then 
removing the blocks and determining their resiliency. The trou- 
blesome skin is placed at the bottom, and a gradually increas- 
ing weight is applied to the top of the jelly block, until a definite 
compression of the jelly is produced. The jellies are cast in 
round brass caps 6 cm. high, 5.5 cm. in diameter at the open top 
and 5 cm. in diameter at the bottom, which is closed with a 
tight-fitting external friction cap. The truncated cones thus 
formed should be exactly 4.5 cm. high, the cups being filled 
only to that level. If the jellies do not push out readily on 
removing the cap, the closed cup may be dipped for an instant 
in hot water. After removal the jellies are placed 1 in a thermo- 
stat until they reach the desired temperature. 

S. E. Sheppard ** states that all modifications of the “shot test” 


10U. S. Pat. 911,277; see J. Soc. Chem. Ind. 28, 252 (1909). 

toa J, Ind. d Hng. Chem. 14, T06 (1922). 

11 J, Ind. Eng. Chem. 12, 255 (1920). 

122 United States Dep’t of Agriculture, Madison, Wisconsin, in Technical Note 
F-32. 

#83. S. Pat. 882,731; see J. Soc. Chem. Ind. 27, 459 (1908). 

14 Sheppard, Sweet, and Scott, J. Ind. Hng. Chem. 12, 1007 (1920). 


176 GLUE AND GELATIN 


err because the stress applied affects both elasticity of the bulk 
and elasticity of the figure. For more exact results he devised a 
torsion dynamometer which measures the force required to twist 
to the breaking point jellies which had been chilled for 3 hours 
at 0° C. Above 10° the jelly strength begins to diminish rapidly, 
though no material change occurs till then. Using this instru- 
ment, Sheppard found that no simple relation holds between the 
concentration of gelatin and the jelly strength; and that jelly 
strength values determined for a single arbitrary concentration 
give a very arbitrary comparison of the jelly strengths, because 
the curves relating these values for different concentrations of 
commercial gelatins are not of a common family and often cut 
each other. He concludes that there is no definite relation be- 
tween the jelly strength at a given concentration, and the glue- 
joint or tensile strength of a dry glue joint.. While the H-ion 
concentration affects jelly strength, there is no simple relation 
between the two.t® 

C. R. Smith 1® describes a simple jelly-testing device which 
may be rigged up in any laboratory. An 80 mm. short-stemmed 
funnel accurately formed to a 60° angle, is closed at one end, 
and 120 grams of mercury are poured in, forming an upper sur- 
face 3 cm. in diameter. Over the mercury is layered 50 cc. of 
gelatin solution, which is allowed to set in a horizontal position 
(fixed by a spirit level) in a constant temperature bath at 10° C. 
The mercury is then run out, the funnel is connected with a 
water manometer, and a reduction pressure (6 dm. of water) is 
produced. The depression of the upper surface of the jelly 
produced by the suction is measured with a micrometer and 
indicates the jelly strength. Smith’s figures show, however, that 
the jelly strengths thus determined do not exactly parallel those 
estimated by his polariscopic method. This consists in finding 
the increment in the specific rotation of a 3 grams per 100 cc. 
solution between 35° C. and 10° C. 

Most commonly the jelly strength is determined by the finger 
test against standard samples, for this method is speedy and of 
sufficient accuracy for commercial purposes. Besides it fits in 
readily with other tests and does not. require any special ap- 


18 Sheppard and Sweet, J. Am. Chem. Soc. 438, 589 (1921). 
16 J, Ind. Eng. Chem. 12, 878 (1920). 


e 


TESTING AND GRADING 177 


paratus. But it demands the possession of standard glues for 
comparison. ‘These, however, may be obtained by careful selec- 
tion, aided by any of the mechanical devices. 


Viscosity or Running Test. 


A viscosity determination in the form adopted by J. Fels ‘% 
is largely used in Germany.'® Fels originally proposed taking 
the viscosity of a 15 per cent. solution of glue at 30° C. with the 
Engler viscosimeter; but finding that some very high test glues 
would not flow at this temperature, later he 1® raised the testing 
temperature to 35° C. 

Fels’ test, as it is sometimes known, is therefore the viscosity 
of a 15 per cent. glue solution at 35°, as fixed by the Engler 
viscosimeter; and it is a single test of great importance. It is 
interesting to note that the temperature finally fixed by Fels 
is that recently shown by C. R. Smith ?° to be the one at which 
incipient gel formation begins, as is evidenced by the polariscope. 
R. H. Bogue *! has also recently recommended what he terms a 
“melting point” determination, which he says gives “a truer 
evaluation of product than by the use of the old and time-hon- 
ored methods.” Curiously enough Bogue’s method, though 
evolved independently after numerous experiments, is practi- 
cally the same as that of Fels. Bogue advises the use of 
the MacMichael viscosimeter?? to determine the viscosity 
(“melting point”) of a 30 to 100 solution of glue at 83° F. 
foe GC.) . 

It has been the general practice in glue testing laboratories in 
the United States to take viscosities with a simple pipette at 
temperature more closely approaching those at which the glue 
is ordinarily used, and with concentrations varying from 10 to 
25 parts per 100 of water, depending on the strength of the 
glue. J. Alexander ?* adopted as standard a pipette of the fol- 
lowing dimensions: 


7 Ohem. Z. 21, 56 and 70 (1897). 

1% See J, Rudeloff, Mitt. K. Materialpriifungsamt 36, 2 (1918). 

18 Chem, Z. 25, 23 (1901). 

20 J. Am. Chem. Soc. 41, 1385 (1919); J. Ind. d Eng. Chem. 14, 485 (1922). 

2 Ohem. Met. Eng. 23, July, 1920. 

22See Winslow Herschel, J. Ind. Eng. Chem. 12, 282 (1920). 

2 J. Soc. Chem. Ind. 25, 158 (1906) ; ‘“‘Allen’s Comm. Organic Analysis,” 4th 
ed., Vol. 8, p. 605. 


178 GLUE AND GELATIN 


Capacity: Rio. ve Corie Si eee ster te ee 45 ce. of water at 80° C. - 
Internal diameter of effuent tube... 6 mm. 

External diameter of effluent tube... 9 mm. 

Over-all length of effluent tube..... 7em, 

Smallest diameter of outlet (about).. 1.5 mm. 

Outside diameter of bulb........... 3 cm. 

Dengthyor-pulbg acy soi nee eee 9.5 cm. 

Length of upper tube........ Detects 22 vGIn: 


This pipette should permit the efflux of 45 cc. of hot water at 
80° C. from the bath in which the glue testing glasses are im- 
mersed, in exactly 15 seconds. The viscosities of glues vary 
greatly as will be seen from the table showing the viscosities and 
jelly strengths of the glues chosen as standards. 

Considerable care is necessary to make pipettes that will give 
concordant results. The size and form of the outlet hole, and 
the length and diameter of the effluent tube are the main factors 
controlling the time of delivery. The efflux hole is made by 
cutting the effluent tube square across, and holding it pendant in 
a Bunsen flame with constant rotation. As the glass softens — 
the hole gradually draws together, and after a few trials can be 
brought to the desired size. It is necessary to have the lower 
graduation point just about where the effluent tube joins the 
bulb, for with very viscous glues there might otherwise be much 
uncertainty due to dribbling of the last few drops. 

The time of efflux is taken with a stop-watch, and care must 
be used to see that no particles of paper, wood, dirt, undissolved 
glue, or glue slime clog the outlet even for an instant during a 
determination, at the conclusion of which the pipette is washed 
out with hot water from the bath. 

A refinement is to keep the pipette in a simple water jacket 
thermostat while running; in this case a glass stop-cock or a 
rubber tube and pinch-cock is used to control the pipette. They 
impede rapidity of work without corresponding increase in ac- 
curacy. More complicated viscosimeters like the Rideal-Slotte 7+ 
though more accurate than a simple pipette are not practical for 
routine work where speed is essential. 

The results of R. H. Bogue (loc. cit.) indicate that the viscosity 
test alone, would correctly classify about 75 per cent. of all 
glues—that is viscosity determined at about 60° C. As E. 

*4 J. Soc. Chem. Ind. 10, 615 (1891). Bogue (‘‘The Chemistry and Tech- 


nology of Gelatin and Glue,” p. 384) describes many of the various viscosimeters 
that have been suggested. 


TESTING AND GRADING 179 


Sauer 7° points out, glue is not a pure substance, but may con- 
tain impurities having a viscosity of their own, or substances 
which materially affect the viscosity of the glue. He concludes 
that “viscosity measurement is no absolute method for deter- 
mining the quality of a glue, especially when dealing with 
products of different origin, which according to their raw ma- 
terials and methods of making may contain more or less foreign 
material and which therefore cannot be compared with each 
other. For practical purposes, however, it is important, espe- 
cially if certain other characteristics be taken into considera- 
tion.”” Sauer here speaks of Fels’ test at 35° C.; he says that 
viscosity 1s good for factory control purposes if foreign addi- 
tions, etc., remain unchanged. 

H. J. Watson,?® while placing most reliance upon jelly 
strength, found it necessary in many cases to take the viscosity 
as well, by Fels’ method. Trotman and Hackford 2” also regard 
jelly strength as the more reliable physical test, although they 
mistakenly place more reliance upon chemical tests. 


Water Absorption Test ** (Schattermann’s Immersion Test). 


A known weight of glue or gelatin is soaked 24 hours in water 
at room temperature. ‘The excess water is drained off, and the 
amount absorbed estimated by weight. High test glues absorb 
from 10 to 15 times their weight of water, weaker glues take up 
only 3 to 5 parts, and very weak glues may actually go into 
solution, forming a slime. Considering the many factors in- 
fluencing water absorption (see p. 87), it is obvious that this 
test can give only a crude approximation as to value. It is, of 
course, impossible to apply it to finely broken or ground glues 
or gelatins. | 


Hygrometric Test (Cadet’s Test). 


This practically obsolete test is based upon the amount of 
moisture absorbed by a glue exposed to damp air, and is even 
less reliable than the preceding. 

Where a glue or a glued article is to be exposed to a damp 


2 Kolloid Z. 17, 1380 (1915). 

26 J. Soc. Chem. Ind. 23, 1189 (1902). 
27 J. Soc. Chem. Ind. 23, 1072 (1902). 
28 Dingler’s J. 96, 119 (1845). 


180 GLUE AND GELATIN 


climate, some form of hygrometric test, simulating conditions 
of service, is desirable. An unpublished report by E. Bateman 
and G. G. Town of the Forest Products Laboratory (U.S. Dep’t 
of Agriculture), Madison, Wisconsin, indicates that at about 
30 per cent. moisture content glue becomes weaker than wood, 
and that above 33 per cent. moisture results in molding, against 
which no harmless preservative has been found. Hygroscopic 
salts greatly increase the water absorption, whereas tanning 
agents seem to decrease it. In Technical Note F-10 of the 
Forest Products Laboratory it is indicated that the moisture 
resistance of animal glues is proportional to the viscosity, jelly 
strength, and grade. High-grade glues absorb water more 
slowly. 5 


Melting Point. 


Glue and gelatin jellies soften gradually when warmed, and 
show no sharp line between solid and liquid. The “melting 
point” will therefore depend upon definition, and is an uncertain 
factor. As a general rule it varies as the jelly strength. 

N. Chercheffsky 7° described a simple apparatus for determin- 
ing the melting point. A 250 cc. beaker is filled with refined 
paraffin oil into which is hung a wire with a horizontal end on 
which are threaded several small blocks of jelly. When these 
lose their rectangular form on warming, the melting point is 
read on a thermometer which hangs as close to them as possible. 
Increased accuracy is had by placing the oil beaker in a EM 
jacket which is gradually warmed. 

Cambon’s fustometer *° consists of a small brass cup which is 
held to a brass rod by a jelly made by dissolving 10 grams of 
the glue or gelatin under test in 40 cc. of water. The brass cup 
is hung at the surface of a beaker of water which is slowly 
warmed, and the melting point is taken as the temperature of the 
water when the brass cup drops off. A cane ferrule weighing 
about 7 grams will serve as the cup. 

A. W. Clark and L. Du Bois* propose to determine the per- 
centage of glue or gelatin which just maintains a solid jelly at 


2 Chem. Z. 25, 413 (1901). 

*°Cambon and Bergmann, Monit. Scient., June, 1907; J. Soc. Chem. Ind. 26, 
7038 (1907). 

31 J, Ind. and Eng. Chem. 10, TOT (1918). 


mee SING AND GRADING — 181 


10° C., but this is a troublesome method and the work of Shep- 
pard indicates that it is not dependable. C. F. Sammet ** deter- 
mines roughly the comparative melting point of glues by placing 
their jellies on an inclined brass plate, which is warmed by 
dipping one end in hot water. The weaker glues melt and slide 
down the plate more rapidly than the stronger ones. Bogue’s 
suggestion (loc. cit.) is to continue the low temperature viscosity 
curve so as to determine, by extrapolation, the point where the 
solution would cease to flow, i.e. the viscosity would reach 
infinity. 

H. Bechhold and J. Ziegler ** propose to determine the melting 
point of jellies by observing the temperature at which 5 grams 
of mercury breaks through. To prevent skin-formation from 
interfering with this test, it would seem wise to protect the 
upper surface of the jelly with a layer of oil or wax, which may 
be removed later on. J. Herold ** observed the temperature at 
which a thin tube containing jelly dropped from a thermometer 
about which the jelly had set. 

Sheppard and Sweet * describe an apparatus which serves to 
measure both melting point and setting point. An intermittent 
stream of air bells (bubbles), under constant pressure, is passed 
through the test solution whose temperature is lowered or raised 
by a surrounding bath. The setting point is fixed as the tem- | 
perature at which the bubbles cease to pass, and the melting 
point as the temperature at which they begin to pass. They 
also describe a simpler melting point apparatus consisting of an 
annular brass weight which slips over a thermometer and rests 
on the jelly by three equidistant wedge-shaped feet. The ther- 
mometer is centrally imbedded in a wide tube of jelly, the bulb 
being just below the surface, and the melting point taken as the 
temperature at which the weight sinks just above the feet. 

The results of Sheppard and Sweet show that melting point and 
setting point are not identical, and also that, as they had pre- 
viously found with jelly strength,** the concentration of the 
gelatin solution is a factor which may of itself change the order 
of grading. Furthermore the order obtained by melting or 


82 J. Ind. and Eng. Chem. 10, 595 (1918). 
33 Z, physik. Chem. 46, 110 (1906). 

4 Chem. Z. 35, 93 (1910). 

35 J, Ind. Hng. Chem. 18, 423 (1921). 

86 J, Ind. Eng. Chem. 12, 1007 (1920). 


182 GLUE AND GELATIN 


setting point determinations may not coincide with the order of 
grading according to jelly strength. 


Setting Point. 


The determination of the setting point or temperature of 
gelatinization, as proposed by K. Winkelblech *’ is just as diffi- 
cult and uncertain as that of melting point. The apparatus of 
Sheppard and Sweet has been referred to above. C. R. Smith 
likewise fixed the setting point by the air bubble method, using 
his polariscope tube. 


Strength Test, also called Shear Test, Joint Test, etc. 


Since glue is used very largely for gluing wood, and since its 
binding strength on wood is a good indication of what it will 
do on other service, it is only natural that a large number of 
strength tests have been described and recommended. ‘The trou- 
ble with them all is the great difficulty of obtaining concordant 
results because of the many variable factors involved. Some 
authors naively discard from their averages results that are 
from 100 to 300 per cent. too low, which shows that one single 
test may be very misleading. While impractical as a routine 
laboratory test in a glue factory, the strength test is valuable, 
and has been largely used to check other tests on glues for air- 
plane propellers, etc. 

R. H. Bogue (loc. cit.) points out one highly important source 
of error in this test, namely the joining pressure. He found that 
the strength of a glued joint varies directly as the joining pres- 
sure applied, up to about 1,000 lbs. per sq. in. Below 200 lbs. 
per sq. in. the variation is large, but beyond that it is small. 
S. Rideal ** tried to avoid the uncertainty due to the variability 
of wood, by using porcelain blocks. Further variable factors are 
the conditions of drying, the amount of glue spread, temperature 
and moisture content of the wood. 

The old method of the Konigliche Artillerie-Werkstatt at 
Spandau was radically defective in that the glue solution to be 
tested was first boiled down to % of its original weight, which 
naturally hydrolyzed the glue but indicated what it would do if 


37 Z. angew. Chem. 19, 1260 (1906). 
88 “Glue and Glue Testing.” 


TESTING AND GRADING 183 


mishandled by workmen. Among the various other methods 
may be mentioned those of Rudeloff,?® who uses red beech blocks, 
and of A. H. Gill,’ who tests maple briquet-shaped blocks in a 
cement tester. Gill also tried unsuccessfully paper impregnated 
with glue in the Mullins paper tester which recalls the similar 
test of Setterberg,*! and briquets of sawdust, sand, fullers’ earth, 
ete., which recalls the work of pe ca bne ch o2 a used impreg- 
nated rods of plaster of Paris. 

P. A. Houseman ** used straight ee walnut wood. G. 
Hopp ** dissolved and redried the glue, cutting it then into strips 
of definite size which were tested for strength and stretch in a 
tensile machine. One hide glue showed an average tensile 
strength of 13,240 lbs. per square inch, while another glue showed 
8,523 lbs. 

The method of the Forest Products Laboratory *° used in test- 
ing airplane glues is as follows: 

“Two blocks of hard maple, about 1 x 2.x 12 inches in size, are 
glued together lengthways along their flat grain, and after stand- 
ing about a week to dry out the glue, are each cut into four 
shear specimens having a glued area of 4 square inches. The 
shearing pressure to separate the blocks is then noted by a testing 
machine, and the percentage of wood torn out by the glue is 
estimated. 

If the failure occurs entirely in the glue, a measure of the 
strength of the glued joint is obtained, but if the failure is 
entirely or partly in the wood, as frequently happens, the full 
strength of the glue is not developed, and the test may have to 
be repeated, using stronger blocks. 

The same method has been used in securing data on the 
strength of wood in shear. Consequently when the strength of 
glue has been determined it can be compared with that of any 
wood whose average shearing strength is known. 


39 Mitt. K. Materialpriif. 36, 2 (1918) ; J. Soc. Chem. Ind. 37, 743A (1919). 

40 J. Ind. Eng. Chem. 7, 102 (1915). 

41 Schewed. teknisk Tideskrift 28, 52 (1898) ; Chem. Z., 1898, p. 283. 

#2 Dingler’s J. 152, 204. 

#8 J. Ind. Hng. Chem. 9, 359 (1917). 

44. J, Ind. Eng. Chem. 12, 356 (1920). 

“See their Bulletin No. 66, Washington, 1920; also Mech. Eng. 41, 382 
(1919). <A detailed description of this test and of several other similar official 
tests, is given by Clyde H. Teesdale in his book with C. Mortimer Bezeau, en- 
titled ‘Modern Glues and Glue Handling,” The Periodical Publishing Co., 
Grand Rapids, Mich., 1922. 


184 GLUE AND GELATIN 


Four specimens are usually broken and an average taken of 
their individual values. The variation: in the values can be 
kept at a minimum if the specimens are selected, prepared, and 
tested under as nearly the same conditions as possible. A very 
important factor is the selection of the wood. ‘The species 
should be the one upon whieh it is proposed to use the glue, or 
one at least equally strong. Hard maple is the standard wood 
in use at the Forest Products Laboratory. Other woods of equal 
or greater strength which might be used are sweet birch, black 
locust, flowering dogwood, canyon live oak, persimmon, big 
shell-bark hickory, and western yew. 

It is a good plan to test hide glue at three or four different 
dilutions. Four different sets of specimens should therefore be 
prepared, using 2, 214, 244 and 234 parts water, respectively, to 
1 part of glue. An exceedingly high-grade glue may work best 
at three to one, and there are low grades which will give best 
results with less than two parts of water to one of glue. Other 
types of glue should also be tested under conditions which will . 
permit them to develop their full strength. 

On account of the variable nature of wood and the impos- 
sibility of doing perfect gluing, the test is far from perfect as | 
-an absolute measure of the strength of a glue, but no other 
strength test has been found to be nearly so good. It merely 
gives an idea of the ability of the glue to hold wood together. 
If only one or two specimens are tested, the results are apt to 
vary widely and be misleading, so it is desirable to base conclu- 
sions upon data from a considerable number of tests. 

As a means of judging whether the glue is being used to the 
best advantage, the shear block test is very valuable. The 
specimens can be prepared from almost any piece of glued work, 
provided the laminations are not thinner than about one-fourth 
of an inch and the grain in adjacent laminations runs parallel. 
It is preferable that the specimens be cut to the size (4 sq. in.) 
but it is not absolutely necessary; smaller sizes can be used if 
conditions require. 

The highest grades of animal glue are the strongest glues used 
in wood working. Their strength is greater than that of the 
woods they are used upon, and when they are properly applied 
they are exceedingly reliable, so long as they are not exposed 
to moisture. The certified glue used in propeller manufacture 


TESTING AND GRADING 185 


was sufficiently strong for the highest type of woodworking, but 
still higher grades of glues are obtainable. The certified glues 
were required to have an average shearing strength of 2,400 
pounds per square inch, with a minimum of not less than 2,200 
pounds per square inch. Most of them, however, actually show 
an average shearing strength of between 2,500 and 3,000 pounds 
per square inch. The shearing strength of the lower grades of 
animal glue, such as 114 and less (See p. 191), is somewhat lower, 
but by careful application fairly high values can be obtained 
from them. 

The water resistance of animal glue is low; but the high 
grades, which have high jelly strength, will stand dampness for 
longer periods than the low grades, which have low jelly 
strength.” 

From the results of a large number of shear tests made by 
the above method, using a joining pressure of 200 lbs. per sq. in.*® 
R. H. Bogue’s *** experiments show that the joining strength of a 
glue is a function both of its viscosity (at 60° C.) and its jelly 
strength, but it is directly proportional to its viscosity at 32° C., 
which he mistakenly calls its melting point.**® Bogue’s results 
indicate that the viscosity at about 30°-35° C. is the best single 
measure of joining strength. It is, so to say, a combination 
viscosity—jelly strength figure; for then, as C. R. Smith has 
shown, the glue is just beginning to gelatinize. Bogue also 
showed that on heating a certain hide glue for twelve hours, its 
joining strength dropped from 2,940 lbs. per sq. in. to 1,965 Ibs. 
per sq. in. In his strength tests he used the procedure of the 
Forest Products Laboratory. 

The technique of the British ieee Inspection Depart- 
ment is as follows: 47 

Carefully selected pieces of hard, dry, straight-grained, Ameri- 
can walnut, two inches wide, nine inches long, and three-eighths 


46 Very high pressures are unsafe as the glue may be mostly squeezed out, 
leaving a “starved” joint. The joining pressure in the above test has not been 
accurately defined, which is a defect common to most descriptions of such tests. 

46a Toc. cit.; also J. Ind. Eng. Chem. 14, 435 (1922). 

46> Bogue originally started out to determine the melting point with a series of 
viscosities at successively lower temperatures, plotting the resultant curve, and 
taking the point of infinite viscosity as the setting point. This being very 
laborious, he found that the viscosity taken at 382°C. gave him a figure that 
was relative to the setting point so determined. This figure was therefore 
taken in subsequent investigations in lieu of the true setting point. 

47 See First Report of the Adhesives Research Committee, London, 1922, p. 18. 


186 GLUE AND GELATIN 


of an inch thick, are planed true on the flat face and then toothed 
with a toothing plane having 25 teeth per inch. 

The glue (concentration not stated) is soaked 24 hours at 
room temperature,*® heated a short time between 60° and 
80° C., and allowed to cool to 60° before application. 

Two pieces of warm wood, which have remained several hours 
in a constant temperature oven at 35° C., are glued on the pre- 
pared surfaces by the warm glue solution applied by the finger, 
carefully avoiding the formation of air bubbles. The two glued 
surfaces are then placed together so as to form a one-inch over- 
lap, giving two square inches of glued area, and a pressure of 
400 pounds per square inch is applied for 12 to 18 hours by 
tested box springs. 

After removal from pressure the joint stands three days and 
its breaking stress is then determined in an adapted form of the 
Avery or Buckton cement-testing machine, the mean of four 
tests being taken. 

The Committee comments that this test cannot in itself be 
regarded as an absolute criterion of the value of a glue, since a 
number of disturbing factors arise, such as variations in porosity 
of the wood, in the heating of the glue, in the temperature of 
the wood, the thickness of glue films, in atmospheric humidity 
and temperature, and in application of the test load. If these 
factors are carefully held in mind, and any obviously erroneous 
test rejected, the experimental error will generally not exceed 
10 per cent. 

The British Engineering Standards Association have fixed the 
following standards for 414-inch test pieces: | 


Class Breaking Stress Use 
Propeller glues ...... 1,100 lbs. per sq. in.—Airscrew manufacture. 
CLOSB aie tal tere edo ee 1000 “ “ “ “ _Tmportant stress-bearing work. 
(Sianeo] lee cere. ene © 900 “ “ “ “ __No stress-bearing work. 
and under 


Laboratory Test Series. 


Many people mistakenly attempt to judge glue by its color, 
odor, clearness, fracture, shape, or thinness of flake, etc. Since 
glue is used for a multitude of different purposes, the use for 


48 Obviously thick cake glue was mostly used, for ground glue soaks up 
within an hour or less. 


TESTING AND GRADING 187 


which a glue is intended should always be borne in mind when 
submitting it to test or technical examination. Frequently spe- 
cial tests must be devised which simulate special conditions 
under which the glue is to be used, so that it is obviously impos- 
sible to include all tests in any ordinary laboratory series. 

There is here given, however, a connected series of tests which 
may be conveniently and quickly run. They cover practically 
all that is needed to gauge the value of a glue, especially when 
one has had practical experience with other lots of the same glue. 

Thin blown glasses about 314 in. high and 21% in. in diameter 
are convenient for making these tests. Twenty-five grams of 
each glue to be tested is broken into small pieces and soaked in 
100 ce. of cold water until thoroughly softened. Thick sheet 
or flake glue should soak overnight in a cool place. 

With the glues under examination, there are at the same time 
soaked up a number of glues of known strength (standards) for 
tests of glue are preferentially comparative to avoid the great loss 
of time involved in fixing absolute conditions. It is desirable 
and convenient to use the standards described later (p. 190), 
since they cover the range of glues and gelatins ordinarily met 
with, and are familiar to most American manufacturers and 
dealers, and besides to many others. 

In cold weather or in testing high-grade glues, 20, 15 or even 
10 grams of glue to 100 cc. of water may be used, providing un- - 
knowns and standards are treated alike in all respects. In 
warm weather low-grade glues must sometime be tested 30 to 
100, unless ice is available to chill the jellies. Gelatins are 
usually tested from 3 to 10 grams to 100 cc. of water. 

When the glues are thoroughly softened, the glasses are placed 
in a simple rectangular water bath having a double bottom to 
prevent the glasses from being too close to the flame, and the 
temperature of the glues raised to 80° C., the contents being 
- meanwhile thoroughly stirred from time to time to insure com- 
plete solution. Insufficient soaking or stirring is apt to leave 
some undissolved glue, which vitiates the test. Upon complete 
solution, the following tests are made in the order given. 

1. Reaction. This is determined by a strip of litmus paper 
which is then allowed to adhere to the right-hand edge of the test 
sheet. If the exact degree of acidity or alkalinity is desired a 
separate titration must be made. The degrees of acidity, alka- 


188 GLUE AND GELATIN 


linity, foam, grease, and odor are conveniently noted on an 
arbitrary scale of 1 to 5; thus under acidity, 1 would mean prac- 
tically neutral, 2 would mean slightly acid, 3 would mean fairly 
acid, 4 would mean strongly acid, 5 would mean very strongly 
acid. 

2. Odor. While the glues are being dissolved, or at any 
other convenient time during the test, the odor is noted. This 
gives some indication as to the stock from which the glue was 
made; in fact, the odor was once seriously proposed as a test of 
quality. Decomposition, though often masked by antiseptics or 
essential oils, is readily detected, for decomposed glue or gelatin 
has a peculiarly nauseating odor. Glues are rated as “sweet” 
(1) or “off” (2 to 5). The ordinary stock odor of a glue is not 
an objection, but with food gelatins freedom from all odor is 
. desirable. | 

3. Viscosity. The viscosity is then taken on each sample 
and each standard, by running the hot glue solution from a 
pipette (previously warmed each time by the hot water of the 
glue bath which serves also to wash it out between determina- 
tions), noting the time of efflux with a stop-watch. Any con- 
venient pipette may be used, but Alexander’s standard pipette 
described on page 177 is of convenient size and shape. ‘The 
average time for a viscosity determination with it is about 40 
seconds. Special caution must be used to see that nothing inter- 
feres, even momentarily, with the efflux of the glue solution. If 
anything clogs the pipette it must be cleaned and the viscosity 
run anew. 

4. Grease. A flat camel’s hair brush is dipped in the glue 
solution, worked into a little aniline or pulp color on the corner 
of a piece of hard sized paper. The colored glue is then painted 
out upon the sheet, where whitish spots or “eyes” appear whose 
number is roughly proportionate to the amount of grease present. 
For an exact determination of grease a separate determination 
must be made. 

5. Foam. Beat the glues rapidly with the glass stirring rod, 
using, say, 30 double strokes (across the diameter of the glass 
and back), and then note comparatively how the foam fades 
away or persists. With some glues the foam dies away speedily 
or even instantaneously, and such are rated 1 (foam free). 


‘TESTING AND GRADING 189 


This is especially the case with greasy glues. Other glues are 
rated from 2 to 5 depending upon the persistence of the foam. 

For more accurate estimation of foam, the glues may be re- 
heated after the tests are concluded, and agitated in a small 
bowl by an egg-beater, the foam being measured in mm. after 
the glues have been poured back into their glasses, or into gradu- 
ated cylinders. 

While very undesirable in a veneer glue, foam is advantageous 
in a gelatin used for making marshmallow confectionery. Trot- 
man and Hackford *° give a method for determining foam, which 
is similar to that described above. They found that peptones, 
overboiling, and alkali (which causes hydrolysis) produce foam. 
H. J. Watson *° found that foam was favored by free alkalis or 
alkaline earths, free acid, zinc compounds, overheating, and 
mucin (Rideal) .°* 

6. Comparative Set. The glasses are now taken from the 
water bath and set aside to cool. Note is made of the order in 
which the jellies set, which is usually in the order of their jelly 
strengths. In warm weather, especially with glues of low 
strength or in weak concentrations, the glues are placed in cold 
or even iced water. 

7. Jelly Strength or “Finger Test.” After the glue solutions 
have set or gelatinized, the glasses are arranged in order of the 
strength or firmness of their jellies. This is done by pressing 
the jellies with the middle finger or with two fingers, and noting 
their comparative resiliency. The unknown glues group them- 
selves as equals to or as stronger or weaker than the various 
_ standards used on the test. The standards should be so chosen 
that they cover the range of the glues on test, which may then, 
if necessary, be graded in between the standards. The difference 
between each standard “grade” is divided into ten “points,” and 
the differences fixed in increments of two points. 

The concentration and temperature of the glue jellies must be 
such as to permit the ready detection of small differences of 
jelly strength, which is difficult if the jellies are too stiff. 

With the finger test the personal equation is naturally a factor; 


40 J. Soc. Chem. Ind. 25, 104 (1906). 

50 J, Soc. Chem. Ind. 25, 209 (1906). 

51 Technical Note No. F-9 of the Forest Products Laboratory deals with foamy 
glues. 


190 GLUE AND GELATIN 


but given proper standards, it is speedy and sufficiently accurate 
for commercial purposes. Mechanical devices are apt at times 
to give erroneous results, particularly in attempting to. take the 
jelly strength under “absolute” conditions without the steadying 
effect of standards. Slight differences in temperature, time of 
set, and amount of evaporation or surface skin will make any 
instrument, no matter how perfect mechanically, give variable 
results. But with standards which are treated the same as the 
unknown glues, there is little chance of sertous error. Where 
glues are nearly alike in strength, their jellies may be broken 
up with the fingers to see the difference between them, if any. 

8. Keeping Properties. The glasses are now allowed to stand 
at room temperature to see how the glues resist bacterial attack 
and mold. If it is necessary to know the keeping properties 
under certain conditions (i.e. at greater dilutions, at higher tem- 
perature, if mixed with color or other ingredients) these condi- 
tions must be simulated in a special test. 

9. Appearance of Jelly. Practically all glues are turbid and 
a rough description clear, cloudy, or opaque will answer. Note 
should be made of any flocculent precipitate or sediment. 
“Opaque” usually means that some whitener has been added, 
1.e. oxide of zinc. 

With gelatins the clarity of the jelly is usually a very im- 
portant matter, especially when they are intended for photo- 
graphic or table use. The clarity is best measured against other 
samples of gelatins. S. E. Sheppard *? has described a turbidi- 
meter which may be used to determine the degree of clarity of 
gelatin and other substances. 


Standard Glues. : 

As these form the fixed scale by means of which unknown 
glues are to be measured, their careful selection and preserva- 
tion in moisture tight packages is a matter of great importance. 
In the past, results of glue tests by various investigators have 
not been comparable because they used different glues, different 
methods, and had no standards of comparison. 

Although no official unanimity of standards exists even now, 
for nearly a century American manufacturers have been using 


oJ. Ind. Eng: Chem. 12, 167 (1920). 


TESTING AND GRADING 191 


a series of loosely fixed standards based upon those established 
by Peter Cooper, the well-known philanthropist, who was an 
American manufacturer of glue. And about twenty years ago 
J. Alexander attempted to fix these standards** so that uni- 
formity might prevail and all use standards of the same strength. 
The table below gives sixteen arbitrarily established, nearly 
equidistant grades which cover the range of jelly strength usually 
met with. The jelly strengths were determined by Alexander’s 
jelly tester, and the viscosities by Alexander’s viscosity pipette, 
both previously described. The viscosity figures are based upon 
many years of laboratory experience with glues tested 25 grams 
of glue in 100 cc. of water; but since glues of the same jelly 
strength may vary greatly in viscosity, there is indicated in the 
table reasonable limits for such variation. Opposite each stand- 
ard is the corresponding Cooper grade, and also the grade re- 
cently suggested by Bogue, based upon the viscosity in center- 
poises of an 18 per cent. (dry basis) solution of glue, at 35° C.°4 


(ay 
QQ So ~ 
aes = | OS e. a a9) 
2 Sa oS S S 
a) oes Son ics) QS “2 QS io 373 
5 $85 2a = ty Sef sec 
ae See SS. SS Ss OS wa 
EGU etetis: oa 40 12 — — — — 
HOU aa eh ace 34 8 — — _ —— 
RAG Saihe. 28 5 —— — } — — 
LBD esc 26 3 258 7.014 A Extra 12 
A ea ae 25 1 236 6,691 No. 1 Extra 11 
TIO ess 24 34. 214 6,067 1 10 
RO ee es 23 34 192 5,443 lx * 9 
ioe. 22 34, 170 4.820 1% 8 
SICAL s «5 21 In 148 4,196 136 | 
711 Pie Sean 20 % 126 3,572 1% 6 
ct ene 19 % 104 2,948 158 5 
DOF ess. 18 % 82 2,324 1% 4 
TICES. aetna 17 yy, 60 1,701 1% a 
“| 1614 % _— -— 2 2 
DAL, see 16 7A — -— — 1 
AOE kee 'e 15% Y = — << — 
Water 15 


* Called “fone cross.”’ 


58 J, Soc. Chem. Ind. 25, 158 (1906). 

54 These were kindly furnished by Dr. Bogue in a private communication. 
Their parallelism to the Cooper grades can be regarded only as approximate 
because of the variability between the jelly strength and viscosities of the 
standard samples. Hide glue grades are known as Hy, Hu, etc., bone glues as 
Bis, Bu, ete. 


192 GLUE AND GELATIN 


In the old days, manufacturers or dealers would occasionally 
check their standards by mutual exchange or by purchasing 
some of a known grade in open market. Nothing has been pub- 
lished regarding the origin of the nomenclature of the Cooper 
grades, but from information received it is probable that they 
represent the distance that a certain weighted foot-rule would 
compress a certain bowl or vessel of glue jelly of known concen- 
tration and temperature. A weak glue allowed it to sink 2 
inches, a strong glue only 1 inch; and the intermediate grades 
were measured in eighths of an inch. The instrument must have 
resembled the jelly tester described by Davidowsky (see p. 174). 


C. R. Smith’s Polariscopic Method.*® 


Smith’s own description follows: 

“In grading gelatins or glues polarize 3. g. per 100 cc. at 35° 
to 36° C. in a 2-dm. tube; cool a portion of the solution rapidly 
to 15° (or 10°) and transfer, before the sample has jellied, to a 
cold 1-dm. tube. This procedure avoids contractions in the jelly 
which may produce poor readings. If the samples need clarifica- 
tion, digest the solution with 5 cc. of light powdered magnesium 
carbonate at 30° to 40° C. for one hour or longer, and filter 
until clear, avoiding appreciable evaporation. Occasionally it 
has been found advantageous to add 0.10 g. of ammonium citrate 
to the filtrate to avoid the formation of insoluble calcium com- 
pounds, but this. does not appear to be necessary if the mag- 
nesium carbonate has been used in sufficient quantity and the 
digestion has not been too short. The procedure for clarifica- 
tion outlined has not been found to change the Pole 
results when applied to clear samples. 

“In place of a constant temperature bath the tubes can be 
placed in a large vessel of water in a portion of the ice-chest 
where the temperature ranges between 13° and 16° and left 
overnight. The next day the temperature can be controlled 
for 4 to 7 hours at 15+ 0.4°.. If a constant temperature bath 
is used the tubes may be read at once in the morning. 

“Considering a sample which polarizes —20.5° at 35° C. 
and — 40.0° at 15° C. in a concentration of 3 g. per 100 cc., 


55 J. Ind. Eng. Ohem. 12, 878 (1920). The original must be consulted for 
tables of results on a large number of glues and gelatins, and for any further 
details desired. , 


TESTING AND GRADING 193 


it is suggested that the strength be expressed as 19.5 pomts at 
15° C., the increment in rotation in Ventzke degrees. Referring 
to Table II (omitted here) we see that a 25-point gelatin at 15° 
represents the maximum strength obtained. In factory control 
the jelly strength determinations can be made by the polariscope 
in the progress of the extractions, evaporation, or drying. The 
solutions are diluted to approximately 3 g. per 100 cc., con- 
trolled by rotations at 35° C. The jelly strength at 15° is 
determined as usual and calculations made by simple proportion 
to reduce rotations to the average basis of — 20.5° V. at 35° C. 
An actual test in factory control gave the strength of a first 
extraction as 17 points at 15° C.; after evaporation it was 10 
points. The evaporated extract was mixed with some unevapo- 
rated material, bringing the strength to 11.5 points; after dry- 
ing it tested 11.6 points. These figures obviously represent poor 
extraction, and considerable loss of strength in the evaporator, 
but show no loss from bacterial action in drying. 

“Jelly strength tests made on samples direct and after in- 
cubation for 24 hours at 37° C. show little or no loss in strength 
of nearly sterile gelatins, while those in active state of decom- 
position show considerable loss with the development of bad 
odors. The following results illustrate this: 


fe ptsdl io CG aieole at 16. GC: Odor 


95.0.3 49. Before After After 
NG. per 100 Cc. Incubation Evaporation Evaporation 
CS 2 ee are — 20.3 — 33.4 — 33.8 Sweet 
ee ee — 20.5 — 39.7 — 368 Bad 
oh ks ae — 20.3 — 35.6 — 316 Bad 


“The solutions were filtered through magnesium carbonate 
to clarify. The increase in rotation in No. 1 was probably due 
to evaporation and experimental error. The loss of jelly 
strength in Nos. 2 and 3 was quite pronounced, with correspond- 
ing production of disagreeable odors.” 

In a lot of bone glues the mutarotation at 15° V. varied from 
10.4 to 24.9, and in a lot of hide and sinew glues, from 3.9 to 


; . rotation 15° V. 
Ste 784° rotation 3° V. 


1.55 to 2.14, with the hide and sinew glues from 1.20 to 2.15. 
The polariscope results checked with the jelly strengths, those 
showing the greatest mutarotation having the highest Jelly 


varied with the bone glues from 


194 GLUE AND GELATIN 


strengths and requiring the smallest amounts to produce a jelly 
of certain standard strength. 

In addition to the foregoing laboratory series of tests, it is 
sometimes desirable to determine moisture and ash. 


Moisture. 


From two to three grams of glue or gelatin are roughly granu- 
lated and dried at 110° until constant in weight. If the product 
is commercially dry, the estimation of water is of no practical 
value, for it varies rapidly with atmospheric conditions, and any 
unusual percentage would at once register itself in reduced vis- 
cosity and jelly strength. 


R. H. Bogue ** found that the water content of air dry glues 
varies directly as the jelly strength, being 13.66 per cent. in a 


fairly strong hide glue and 10.68 per cent..in a weak bone glue. 
D. Jordan Lloyd *’ reports that a specimen of Coignet “Gold 
Label” gelatin had 20 per cent. of moisture removable by drying 
6-8 hours in a hot air oven at 110°. 


Ash. 


The ash of gelatin is of importance for in it may be sought 
certain forbidden impurities. Besides, in some jurisdictions, the 
ash of gelatin, if above certain allowed percentages, is regarded 
as an adulteration. No manufacturer would intentionally raise 
the ash, for this would lower the test; but excessive ash may 
indicate careless manufacturing. 

The ash of glue runs usually between 3 and 4 per cent. Some 
bone glues contain considerable calcium phosphate, while hide 
glues are apt to have calcium sulphate or chloride resulting from 
neutralization of the lime used in preparing the stock. In the 
ash also appear various whitening agents such as zinc oxide, 
lead sulphate, or carbonate, chalk, clay, ete. | 

To estimate ash, place 2-3 grm. glue in a large platinum cru- 
cible and heat slowly, as the glue at first intumesces violently. 
Ash at a low redness, preferably in a muffle, using a few drops 
of nitric acid to insure the oxidation of all the carbon. Accord- 
ing to Kissling °* the ash of bone glue fuses in the bunsen burner, 


5 Chem. Met. Eng. 28, 105 (1920). 
5 Biochem. J. 14, 148 (1920). 
88 Chem. Z. 11, 691 and 719. 


195 


TESTING AND GRADING 


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196 GLUE AND GELATIN 


is neutral and contains phosphoric acid and chlorides, while hide 
elue ash is infusible, alkaline, and generally free from phos- 
phates and chlorides. 

Since hide and bone glues are frequently mixed both in the 
liquor and dry form, it is obviously unsafe to draw any con- 
clusion from the ash as to the raw material used. The com- 
position of the ash usually depends upon the process used, and 
its estimation, in glue, unless for some special purpose, is not 
commercially necessary. 


Recording Tests. 


There is given on page 195 a specimen of a laboratory test sheet 
used in recording the connected series of tests just described. 

In 1898 Friman Kahr *® proposed a series of four tests de-: 
signed especially to grade glues for joining use. These were: 

1. Adhesion =the weight of dry glue necessary to make up 
100 lbs. of liquid having the proper viscosity or body at 60° for 
use in making joints. This figure varied from 60 lbs. in the case 
of the lowest grade glues to 29 lbs. with the highest grades. 
It is in effect a viscosity determination. 

2. Economic value = adhesion figure x cost per pound. Thus 
Kahr’s figures showed 29 X 18 = $5.22 per 100 lbs. of liquid 
made with the highest grade of glue (cost 18¢ per pound) and 
60 « 5144 = $3.80 per 100 lbs. of liquid made with the lowest 
grade (cost 544¢ per pound). 

3. Cohesion or strength = crushing strength of the glue jelly 
at the concentration indicated under 1, and determined at 65° F. 
(about 18° C.). This was subsequently fortified by a joint 
strength test. 

4. Congealing test = temperature at which the glue made up 
as indicated under 1, would gelatinize. This figure ran between 
91° and 75° F., and gave some indication as to speed of set. 

One radical defect in this system is the latitude left open in 
fixing the “proper viscosity” under 1. But this is the method 
most consumers of glue follow—they find by rough trial about 
how much glue is needed to make up a ready-to-use solution 
and then figure its cost. Since many tests are needed to estab- 
lish the minimum amount of glue needed, they usually stop when 


5° International Fisheries Congress, ‘Bergen, Norway, 1898 ; also a periodical, 
Glue, published by him at Hast Haddam, Conn. 


TESTING AND GRADING 197 


an approximate figure is reached. In any event Kahr’s tests 
are practically intended for joint work, and cannot serve for a 
general laboratory method intended to indicate the value of 
glues for most ordinary uses. 

R. H. Bogue,®® after reviewing the various testing methods, 
proposes a system of evaluation based on measurement of the 
viscosity of an 18 per cent. dry basis (equal on the average to 
20 per cent. of commercial glue) solution of glue in the Mac- 
Michael viscosimeter at a temperature of 32° to 35°, the latter 
being preferred since there is less change of viscosity with time. 
This is the one primary test; and Bogue’s figures show that it 
correctly classifies glues on the basis of their joining strength. 
Speaking of the jelly strength and viscosity at 60° which the 
primary test is to supersede, Bogue says: “They may, however, 
be of great value in secondary evaluation, i.e. in determining the 
adaptability of a given glue to a given service. For example, 
the jelly consistency would be of value in selecting glue for the 
printer’s rollers, and the rapidity of setting of the jelly as well 
as the. viscosity at working temperatures would be desirable 
data for the wood-working industry.” 

While Bogue’s primary test is the best single test of the value 
of a glue or gelatin for joint work, the fact that the similar test 
of Fels, proposed in 1897, has not met with general acceptation 
indicates that neither is_ self-sufficient. Bogue’s statement 
above shows that the secondary tests are essential. <A glue test 
may be considered from the standpoint of 1, the seller or manu- 
facturer; 2, the buyer or consumer; 3, the chemist or tester. Let 
use review these. 


Tests from the Standpoint of Seller, Buyer and Chemist. 


The manufacturer or dealer receives different lots of boilings 
of glue from the factory, and before offering them to his cus- 
tomers, must test them to find all their principal characteristics 
so that the lots may be graded and selected to be sent out to the 
trades for which they are most suitable, and especially so that 
deliveries to each consumer are uniform in working properties 
and free from objectionable defects. Nothing is more essential 
to the buyer than uniform delweries, and this is therefore equally 


6 J. Ind. Eng. Chem. 14, 485 (1922). 


198 GLUE AND GELATIN 


important to the seller. Even in the absence of absolute stand- 
ards, -the seller keeps reserve samples of his deliveries, against 
which he can test subsequent shipments. He must give his cus- 
tomer glue the same as last. Some wood-workers use the same 
glue for both joint and veneer work. It would be a serious mis- 
take to give such a consumer a glue that would foam in his 
veneering machine, although a slightly foamy glue would serve 
for a. shop where only hand gluing was done. 

As will be seen in the next chapter, entitled “The Uses of 
Glue and Gelatin,” each industry and, in fact, often each con- 
sumer has peculiar requirements. The seller must have all the 
test figures he can reasonably get out of his laboratory test; 
therefore it is better to have a jelly strength and a working 
viscosity figure than it is to have the lower temperature vis- 
cosity figure alone. Besides all the other figures are of primary 
importance, for one objectionable characteristic is enough to 
divert a lot of glue from one industry to another, i.e. greasy glues 
are suitable for veneering (if foam free), but not for paper 
sizing. In matching competitors’ samples it is also wise to 
consider all of its characteristics, for most’ buyers do not give 
specifications and some even fail to give the seller essential in- 
formation or samples. 

The buyer should test his glue to be sure that he is getting 
what he ordered, and that the delivery has no objectionable fea- 
tures. Different lots of glue are seldom identical, and any 
accidental or intentional variation in delivery may cause damage 
and loss—even too good a glue sent in error may do this. Since 
by the exercise of ordinary prudence the buyer may test his 
glue, he cannot hold the seller legally liable for any loss he may 
suffer from its use; but can claim only the difference in value 
between what was delivered and what was paid for. The buyer 
should furthermore be in a position to test small samples of 
glues offered by competing sellers, and by eliminating those 
of least value, minimize the number to be selected for practi- 
cal factory trial. The majority of buyers trust the sellers, 
using the new lot just the same way as its predecessor, and 
their confidence is never intentionally violated by reputable 
sellers. | | 

Most generally complaints made by consumers may be traced 
to causes other than the glue, i.e. grease spots on paper to ma- 


TESTING AND GRADING 199 


chine oil, poor joints to a broken window pane which in winter 
allowed the glue to chill. 

Since the majority of manufacturers and sellers do their own 
testing, the consulting chemist is usually employed by some user 
of glue, or else as an arbitrator or an expert in disputed cases. 
Should his practice in this line be insufficient for him to have 
absolute standards, he can readily obtain from his clients sam- 
ples with which to make comparative tests. 


Chapter 13. 
The Uses of Glue and Gelatin. 


There are certain simple principles which govern the prac- 
tical handling of glue and gelatin, no matter for what purposes 
the various grades or qualities may be used. 

Glue and gelatin should be stored in a place that is neither 
too hot nor too damp. Storage in a hot dry place causes con- 
siderable loss of moisture, and unless due allowance is made for 
this, too much actual glue will be weighed off. (For all strictly 
accurate or scientific work, calculation must be made of the 
actual amount of glue in solution, by drying at 110°C.) 

Dampness is apt to cause the glue to take up so much moisture 
that it will mold and undergo bacterial attack, which injure its 
strength and of course, ruin food and bacteriological gelatin. 
Ground glue and ground gelatin in barrels are less apt to suffer 
from these changes, but to be safe in tropical climates, glue and 
gelatin should be stored in air-tight containers. Even with these, 
trouble may be caused by “sweating.” 

The first essential is to select the grade of glue or gelatin 
best suited for the work in hand. This involves consideration 
of the quality and cost of the product to be made. Some sug- 
gestions as to selection will be given later, but in many cases the 
safest procedure is trial under actual working conditions which 
must be simulated in laboratory tests. 

The second essential is to test carefully each delivery or lot 
to be sure of its uniformity. Any difficulty may then be traced 
to its real cause instead of simply blaming it on the glue. 

The third essential is to establish correct methods and definite 
formulas for preparing and using the product. While formulas 
must differ with each class of work and even with weather con- 
ditions, the following principles should be observed in each ease. 

1. Use definite weights of glue and gelatin. Do not depend 
upon measures, for owing ‘to variations in thickness. of cut, glues 
are apt to vary greatly in specific gravity, even in the ground 

200 : 


THE USES OF GLUE AND GELATIN 201 


product. Definite quantities of water and other ingredients 
must also be used. 

2. Soak the glue or gelatin in clean cold water until it is 
thoroughly softened. Ground glues soak up very quickly and 
are preferable for this reason, besides occupying less storage 
room. They should be stirred into the water to prevent the 
formation of lumps containing dry glue, which will not readily 
dissolve. Thick sheets are usually soaked overnight, and when 
bent after soaking should show no whitish “bone” at the bending 
line, but be thoroughly softened. 

3. Melt preferably in a water or steam-jacketed bath and 
keep at as low a temperature as the work will permit. Use a 
thermometer, and preferably do not heat above about 65° 
(150° F.). Glue loses strength continuously under the action of 
heat, and it is more advantageous to heat up successive small 
lots rather than to have a large lot “cooking” all the time. In 
many cases, e.g. with leather belting cement, flexible padding 
or book-binding glue, etc., large lots are made up and cast into 
pans, the jelly blocks being kept and heated up a little at a 
time. 

4. Make good evaporation from open glue pots. Some users 
who neglect this, fail to notice loss in strength due to prolonged 
heating, because the concentration of the solution due to evapora- 
tion may in a large measure compensate for the deterioration of 
quality. 7 

5. Use clean utensils and keep them clean, and add anti- 
septics if necessary. Glue and gelatin are particularly subject 
to bacterial attack, which speedily destroys their strength, 
usually with the development of a nauseating odor. The pre- 
servatives added to commercial glue are sufficient to preserve it 
only when in fairly strong solutions. Dilute solutions require 
additional quantities of preservatives, among which may be 
mentioned carbolic, cresylic, sulphurous, and boracic acids, borax, 
formaldehyde, sodium bisulphite, etc. Low temperatures as well 
as chloroform and toluol are commonly used to preserve gelatin . 
solutions used in scientific work. 

Glue and gelatin are used for such a multitude of purposes 
that it will be possible to consider in detail only a few of them. 
The principal uses of glue and gelatin classified as to general 
nature, are as follows: 


202 GLUE AND GELATIN 


Adhesive. 


Wood Joints—including furniture, pianos, musical instruments 
(violins), wooden-ware, house-trim, barrel heads, dowels, 
blocks, coffins, toys, etc. 

Veneers—for furniture, house-trim, seats, shipping packages, 
etc. 

Paper Boxes—“setting up” and covering paper boxes, sealing 
cartons, etc. 

Packaging—making and wrapping paper packages on auto- 
matic machines. 

Leather Goods—hbags, belts, pocket-books, dress suit cases. 

Leather Belting—most machinery belts are made with glue 
and many are made “endless” with glue to avoid lacing. 
This is especially desirable on high-speed machinery. 

Bookbinding—for case making, rounding and backing, tabbing, 
etc. 

Padding—making pads, “blocks,” calendar pads, etc. 

Gummed Cloth and Paper Tape—‘passepartout” (for making 
picture frames), “tape” to replace string in sealing cartons, 
packages, etc. 

Fireworks—as an adhesive and to cause the flaring of mixtures 
otherwise explosive. 

Abrasives—in making sand, emery and garnet papers, as well 
as wheels and belts, aie holds the abrasive to the base. 


Glue, gelatin and isinglass are also important components of 
many adhesive mixtures and cements. 


Sizing or Stiffening. 


Paper—including wall paper, ine paper, stocks, bonds and 
government currency. 

Hats—most straw hats, some felt and cotton hats are pA 
with glue or gelatin. 

Textiles—including silk, ribbons, burlap (for wall coating 
linoleum, etc.), shade-cloth. — 

Barrels—for oil, alcohol, turpentine, etc. 

Wood—cheap furniture, reed furniture, including baby car- 
riages, broom and other handles, are sized to prevent absorp- 
tion of too much varnish. 

Walls—before papering or painting. 


_— 


THE USES OF GLUE AND GELATIN 203 


Calsomine—for walls; both hot and cold water “paints.” 

Scenery—after sizing, is painted with distemper colors sized 
with glue. 

Leather—dressing leather, making polishes, etc. 


Compositions. 


Matches—forms and binds the heads and reduces the explo- 
sion to a flare. 

Dolls’ Heads—including flexible “rubber” toy heads. 

Printers’ Rollers—used in practically all printing presses. 

Gas Tubing—the flexible glue is hidden by a cotton tube. 

Spangles—used in decorating dresses. 

Gelatin Foils—used for wrapping, ‘windows’ in boxes, toys, 
decoration. 

Hectographs—the gelatin-glycerin duplicators are sold under 
a variety of names. 

Microscopy—for glycero-gelatin mounts. 

Bottle Capping—made flexible with glycerin. 

Moldings—for picture frames, room molding, etc. 

Plaster Casts—glue is used for making flexible molds in which 
are made large numbers of plaster of Paris casts. 


Colloidal Protector or Colloidizer. 


Electrolytic Refining of Metals and Electroplating, i.e. lead, 
copper, etc. 
Electroplating—to make a fine-grained coherent deposit. 
Making Colloidal Precipitates—i.e. colors, insecticides, phar- 
maceuticals, etc. 
Inhibiting Crystallization—i.e. diminishing the sensitiveness 
of the explosive, lead azide. 
Making Emulsions—i.e. of fats, oils, etc., for insecticidal uses, 
etc. 
_ Stabilizing Foam—1i.e. in Fire Foam extinguisher. 
Photography—gelatin has practically superceded collodion, and 
_ all other colloids in making plates and films, and 1s Jargely 
used in printing out papers. 
Plaster of Paris—glue is used as a retarder. 


204 ~ GLUE AND GELATIN 


Miscellaneous. 

Gelatin Printing—bichromated gelatin is made into a nega- 
tive usable on printing presses. 

Glass Chipping—high-grade glue, in drying, tears the surface ' 
from glass, making a rough “frosting.” Dissolved salts in- 
fluence the nature of the surface. ih 

Rubber—glue is incorporated with rubber and is said to im- 
prove greatly the wear of solid tires and of casings. 

Artificial Silk—one variety is made from gelatin. Recently 
Dr. A. D. Little made some gelatin from pigs’ ears, con- 
verted this into “silk,” from which a purse was knitted. 
He thus accomplished the supposedly impossible task of 
“making a silk purse out of a sow’s ear.” 

Millinery—artificial fruits, and flowers are made with the 
ald of gelatin, which is also used to glue together silk threads 
to form a brilliant “straw.” 

Directing Chemical Change—certain reactions are influenced 
by the presence of gelatin. 

Pharmacy—gelatin is used for coating pills, making capsules 
for powders (e.g. quinine) and oils (e.g. castor oil) ; also to 
make emulsions? and colloidal remedies (e.g. collargol, etc.). 

Bacteriology—gelatin is an important constituent of culture 
media and serves to make ultra filters. 

Medicine—gelatin is used for certain poultices and as an in- 
valid food. It is also used intravenously, although gum 
arabic has superseded it in cases of shock. Formogelatin is 
a wound dressing. 

Food—gelatin is used in confectionery (marshmallows), ice 
cream, jelly powders, and in the household to make various 
desserts. 


Some of the uses of glue and gelatin will now be considered 
in more detail. 


Wood Joints. 4 


No adhesive makes as strong a joint as the higher grades of 
animal glue. When properly made with seasoned wood, they 


‘H. N. Holmes and W. C. Child, J. Am. Chem. Soc. 42, 2049 (1920), have 
made a careful study of gelatin as an emulsifying agent. 

‘4A description of modern glue-room practice is given by C. M. Bezeau in 
‘“‘Modern Glues and Glue Handling,’ by Teesdale and Bezeau. 


THE USES OF GLUE AND GELATIN 205 


are thoroughly reliable if not exposed to undue heat or mois- 
ture, and they are stronger than the hardest wood. Specifica- 
tions for glues to be used in making airplane propellers called 
for an average shearing strength of 2,400 lbs. per sq. in. with a 
minimum of 2,200 lbs., but most of the glues showed actually 
between 2,500 and 3,000 lbs. per sq. in.2, The glue mostly used 
tested about 130, but some factories got satisfactory results by 
careful manipulation of somewhat lower grades.’ 

While the water resistance of animal glues is low, the higher 
grades stand dampness longer than the lower grades. Outside 
of this feature there is no object in using a glue that is much 
stronger than the wood, unless warranted by cost or some other 
factor. 

The wood surfaces should be true with well joined surfaces, 
which are often scratched or slightly roughened to increase the 
total area available* The wood should be dry, seasoned and 
warmer than the room temperature if possible, certainly not 
colder. The glue should be liberally applied and quickly put 
under pressure of the clamps before it gelatinizes, or the joint 
will be weak. Cold wood, a cold room, or a chilly draft (e.g. 
from an open or a broken window) may chill the glue to a jelly 
which has practically no adhesive strength. Too high a pres- 
sure may squeeze out too much glue and give a weak “starved” 
joint. The pressure must be uniformly distributed.® | 

R. H. Bogue found that the shear strength of joints increases 
rapidly with increase in joining pressure up to about 200 lbs. 
per sq. in., after which it increases more slowly to 1,000 lbs. 
per sq. in. The glued joint should remain under pressure 
about 24 hours and must stand about a week to dry out thor- 
oughly, before it develops into its maximum strength. Wood 
when glued should usually show a moisture content of from 8 
to 12 per cent. 

The best proportions of glue and water will vary with the 
gerade of glue. U.S. Navy Specification No. 11-B for hide glue 


2S. W. Allen and T. R. Truax; report No. 66, Forest Products Laboratory, 
1920. 

3See C. R. McKee, “Interpretation of Glue Analysis,’ Bulletin of vhiage 
Section of A. C. 8S. 8, 66 (1921) ; Chem. Abs. 15, 1419. 

The Forest Products Laboratory, Technical Note No. F-5, found practically 
no difference between smooth and scratched joints (made with a tooth plane) 
on hard maple. 

5 See Technical Note No. 92, Forest Products Laboratory. 


206 GLUE AND GELATIN 


certified for use in airplane construction, says that the sample 
shall be mixed with water in four different proportions, namely 
2 to 1, 214 to 1, 2% to 1, and 234 to 1, by weight, and the pro- 
portion at which the greatest strength is indicated will be used 
in judging the strength of the glue.® 

The consensus of opinion among experienced woodworkers is 
that hide glues running in jelly strength from about 70 to 130 
are most satisfactory for joints. Some bone glues give good 
results, Bogue’s results showing some that gave a joint strength 
of over 2,100 lbs. per sq. in. Much depends upon the 
quality of the bone glue, the feeling against it being partly due 
to the fact that most of the bone glue sold is low grade. 

Where great strength is not essential, bone and hide mixtures, 
and even straight bone glues may be used for joints. 


Veneers. 


Since veneers undergo little strain, have large surfaces, and 
are usually protected by paint or varnish, a lower grade of glue 
may be used for them than for joint work. Hide, bone or mixed 
glues testing from about 50 to 70 are usually employed. Higher — 
test glués are apt to give trouble by setting too quickly, though 
this is not a factor where hot cauls are employed. Besides 
higher grade glue is more resistant to moisture.’ 

Since most veneers are glued on a machine with revolving 
rolls, freedom from foam is essential; for foam means blistered 
or loose veneer. 

Overheating the glue, or running the spreader idle or too 
rapidly may cause foam. Among the foam preventatives used 
are milk and soluble oils and fat emulsions. 


Paper Boxes. 


' For “setting up,” quick setting glues testing from 70 to 90 | 


are best. For “covering” or “stripping,” low grades testing from 
30 to 60 are used; automatic machines work best with the higher 


® See also “Selection and Testing of Animal Glue for High Grade Joint Work,” 
by G. M. Hunt and W. L. Jones, Forest Products Laboratory, 1920. 

7™ Technical Note No. F-10 of the Forest Products Laboratory gives the 
results of moisture tests on veneers. A high-grade glue stood 98 per cent. 
humidity at 80° for 198 hours as against 24 hours for low grades. 


THE USES OF GLUE AND GELATIN 207 


grades. The ratio between cost and water-taking capacity 
usually influences the selection. When the boxes are used for 
silver-ware freedom from tarnish-producing sulphur compounds 
is essential. 


Leather Belting. 


Here strength, flexibility and resistance to moisture are essen- 
tial. The belt cements usually employed are mixtures of high- 
grade glue or gelatin, with glycerin and some antiseptic. The 
highest test glue (130 to 160) is preferable as less of it remains 
in the finished joint, and it is more resistant to moisture. 


Sizing and Stiffening. 


The “crackle” of new banknotes is produced by sizing them 
with a solution of high-grade (test about 150), light-colored 
glue to which alum has been added. For tub-sizing writing 
paper, to give a smooth surface which will hold the ink, a light- 
colored glue is essential; the stronger it is the more dilute it can 
be used. About 2-4 per cent. of alum is used with the glue, and 
the sizing increases the strength of the paper materially. Glue 
can also be used to size paper in the beater. EH. Heuser ® reports 
that precipitation methods were unsatisfactory, but upon adding 
sufficient talc, the glue was adsorbed and held by the paper. 
In making wall paper, the glue solution serves to “free out” or 
deflocculate the clay and color with which it is mixed, as well as 
to bind it to the paper. Any sweet free-flowing foam-free glue 
may be used, grades testing about 60 being most employed. 
In sizing barrels, it is essential that the glue should not melt 
even in hot weather, while its jelly is being dried out inside the 
barrel. In the turpentine belt, a concentrated solution of cheap 
glue is used, but where care and attention prevail, the best and 
most economical results are had with high-grade glues, the melt- 
ing point of the jelly being sometimes raised by the addition of 
chemicals. Gelatin is largely used to stiffen straw hats, but 
many factories get good results with light-colored glues of 
medium strength. 


8 Papier Z. 41, 1865 (1916). 


208 GLUE AND GELATIN 


Compositions. 


Many plastic compositions are made with the use of glue as 
a binder. High grades (120-160) are most often employed, and 
they are often made insoluble with formaldehyde, etc., and 
rendered flexible by glycerin. For printer’s rollers the highest 
grades of glue (140-160) are used, and a similar flexible gelatin- 
glycerin composition is used for hectographs and similar duph- 
cators. Flexible glues used for book-binding, padding and the 
like consist mainly of glue, glycerin and preservatives, as also 
does flexible gas tubing composition. 

Gelatin foils and spangles are made by drying gelatin solu- 
tions on level sheets of plate glass. The solutions are often 
colored or treated to give beautiful surface effects and usually 
contain some glycerin. 

In making plaster of Paris casts, only the highest grade of 
glue or gelatin should be employed. If too weak the glue mold 
may soften from the heat of the setting plaster, or break, espe- 
cially when being pulled off from “undercut” work. 


Photography.° 


Chemical analysis and physico-chemical tests are subordinate 
to actual trial of the gelatin in emulsions, which is depended 
upon to show its ability to develop the purely photographic 
qualities of the sensitive materials. But the chemical and physi- 
cal tests are not only helpful but indispensable in judging how 
the gelatino-silver halide emulsions will behave during develop- 
ment, washing, fixing, after-treatment (hardening), drying; also 
in judging its transparency, freedom from color, from objection- 
able impurities, from excessive acidity or alkalinity. 

The tests suggested are the following: 


PHYSICAL TESTS. 


Jelly Strength. For most purposes, a high jelly strength © 
(130-160) equal to or greater than the highest Cooper grades, 
is. necessary, although for some purposes gelatins of lower jelly 
strength may be used. The jelly strength test is generally more 


® This is based on information kindly supplied by Dr. S. E. Sheppard, of the 
Eastman Kodak Company. 


THE USES OF GLUE AND GELATIN 209 


useful than the melting point, which has been used to classify 
gelatins as “hard” and “soft.” 

Viscosity. A 6 to 10 per cent. solution is prepared under care- 
fully standardized conditions, and the viscosity determined 
(preferably in centipoises) at 100° F. and 150° F. 

Melting Point and Setting Point. These vary materially ac- 
cording to the method used. Using Sheppard’s apparatus (see 
p. 175) and 10 per cent. solutions, “soft” gelatins set at from 
19° to 28° C., “hard” gelatins at from 23° to 27° C. The melt- 
ing points of these solutions are 2° to 3° higher. 

All these tests should be checked by comparison with the ash 
analysis, and a determination of the hydrogen ion concentration 
a value), which fixes the effective, as opposed to the total 


acidity. Photographic gelatins usually have a Py of between 5 
and 6, the generally allowable limits being 4 and 7. The Pu 


value greatly affects the physical behavior, as also does the 
presence of aluminium salts. 

Clarity, color, odor, and appearance of the gelatin are, of 
course, always noted. 


CHEMICAL TESTS. 


Moisture. The usual limits are 8-15 per cent. More than 
15 per cent. means bad keeping properties and lability to bac- 
terial infection, whereas an over dried gelatin is likely to cause 
trouble. | 

Ash. Though gelatins with 3 per cent. are sometimes used, 
2 per cent. is a safer limit. Within these limits, CaO, Na,O, 
K,O, SO,, Cl and P,O, are harmless. Iron and copper should 
not exceed 50 to 60 parts per million; lead not over 50 parts per 
million. Al,O, should not exceed 0.2 per cent. of the weight of 
the dry gelatin. (C. R. Smith ** found that a bromide emulsion 
made with ash-free gelatin was transparent and could be 
“cooked” indefinitely without ripening. He concludes that ripen- 
ing is controlled by the ash constituents. They evidently op- 
pose the protective action of the gelatin——J. A.) 

Sulphur dioxide (determined by distillation into iodine) should 
not exceed 0.1 per cent. 

sa J, Am, Leather Chemists’ Assoc., Oct., 1922. 


210 . GLUE AND GELATIN 


Ammonia. Not more than a trace should be present. 
Acidity or Alkalinity. Determined by titration of a 2 per. 


cent. gelatin solution with 5 KOH or co HCl, using phenol- 


phthalein as an indicator. | 

Reducing Substances. Generally, photographic gelatins should 
not reduce cold ammonical silver nitrate solution in the dark, 
within 12 hours. To be of much utility this test must be worked 
in conjunction with emulsion trials. 

Grease. The usual test (see p. 188) for “eyes” or “comets 
is made and the finely powdered gelatin may be extracted with 
benzene. Only a negligible amount should be present. 

Mucin, etc. A 2 to 5 per cent. solution should give no pre- 
cipitate when acidified with acetic acid (to about 3 per cent.). 
One to 2 per cent. of alum or chrome alum should cause no pre- 
cipitate. 

Inhibiting Crystallization. In the manufacture of lead azide 
(PbN,) which is used as a “starter” for explosives, some glue, 
gelatin or dextrine, is added to the reacting mixture of sodium 
nitride and lead nitrate to prevent the formation of normal 
crystals of the lead azide which tend to explode spontaneously, 
probably by fracture.®> 

Directing Chemical Change. The presence of glue in a reac- 
tion mixture may change. entirely the nature of the reaction 
and of the end products. Thus Kohlschiitter?® says: “In 
aqueous solution the action of chlorine on ammonia leads speedily 
to complete decomposition with the evolution of nitrogen; on 
the other hand an addition of glue brings about the initial 
formation of chloramide, which because of the increased vis- 
cosity of the solution has a chance to form hydrazine with a 
second molecule of ammonia, so that the process follows one 
or the other of the following reaction equations: 


With addition of glue 
NH, + Cl, —> NH,Cl-+ HCl 
NH,Cl + NH, —> NH, .NH, + HCl 


In aqueous solution 
2NH, + 3Cl, = N, + 6HCl 


*” See A. G. Lowndes, Kolloid Z. 28, 288 (1921). 
10V. Kohlschiitter, ‘‘Die Erscheinungsformen der Materie,’’ Berlin, 1917, p. 


9 


300. 


THE USES OF GLUE AND GELATIN 211 


This explains the results of H. Raschig, who, applying some 
principles of Pope and Barlow, added glue to the solution of 
hypochlorite and ammonia, and increased the yield of hydrazine 
from a few per cent. to a commercially possible yield of 40-60 
per cent. 

Bacteriology. Gelatin is largely used in bacteriology as the 
chief ingredient in many culture media, for the exact prepara- 
tion of which the reader is referred to books on this subject. 
Certain classes or kinds of bacteria may be distinguished by the 
way they act on the jelly; some fluidify it. Only the purest 
gelatin should be used for this work, because metallic impurities 
may exert an effect of their own on bacterial growth. This 
effect need not necessarily be an inhibitive one, for minute per- 
centages of antiseptics may act as stimulants. Still the bac- 
teriologist should be as free from disturbing factors as possible. 

Leffmann and La Wall ** found two samples of imported bac- 
teriological gelatin with 265 and 835 parts per million of SO,, 
and they believe that such gelatin is unsuited for bacteriological 
work. P. Poetschke,'* however, found that on preparing nutrient 
media with a gelatin containing 0.1108 per cent. SO., the nutri- 
tive solution after heating under pressure, contained only 0.0009 
per cent. SO, instead of the 0.0133 per cent. required by calcu- 
lation. ‘That is, 93 per cent. of the SO, was lost in the process 
of preparation, which may account for the fact that bacteri- 
ologists have had no trouble on this score. 


Formogelatin. 


Very small amounts of formaldehyde (less than 1 in 10,000) 
do not exert an appreciable effect on gelatin solutions apart from . 
a preservative action. With increasing amounts of formalde- 
hyde, especially with concentrated gelatin solutions or those 
containing free alkali, the gelatin is converted upon drying into 
an insoluble substance known as formogelatin, which seems to 
be an adsorption compound.?”* Acrylic aldehyde is said to pro- 
duce a similar product, but acetic aldehyde reacts only in the 
absence of water. | 

1 Analyst 36, 271. 


122 J, Ind. Eng. Chem. 5, 980 (1918). 
12a See Allen’s “Comm. Organic Analysis,” 4th ed., Vol. 8, p. 600. 


212 GLUE AND GELATIN 


The rubbery but rather brittle jelly of formaldehyde-treated 
gelatin is insoluble in cold or in boiling water, but in contradis- 
tinction to the corresponding casein compound, it dissolves upon 
treatment with dilute (1.34 sp. gr.) sulphuric acid for 12 hours. 

When dried and powdered, formogelatin is used in surgery as 
an antiseptic dusting powder for wounds. In preparing it, any 
free acidity in the gelatin is neutralized (e.g., by agitation with 
calcium carbonate), and any trioxymethylene formed is dissolved 
out with boiling water which does not affect the formogelatin. 
If unaffected gelatin be present, the filtrate will gelatinize, espe- 
cially when chilled. 


Gelatin as a Food. 


The value of gelatin in food products is threefold. In the 
first place its peculiar physical properties enable it to give a 
desirable body, stiffness, or texture to many foods. Gelatin 
jellies are used in almost every home, and those who know its 
value, use gelatin in preparing ice-cream, charlotte russe, 
Bavarian creams, and many other desserts. Gelatin colored red 
is also used to garnish meats, and gelatin acts the part of a 
binder in aspics, head-cheese, and cold meat or fish served “en 
gelée.” 

A second function of gelatin in many foods is to render them 
more digestible by virtue of its action as a protective colloid. 
This applies especially to milk and milk products, such as ice- 
cream. About 14 per cent. of gelatin had long been used both 
by cooks, housewives and by practical ice-cream manufacturers 
who knew that it gave the ice-cream a smooth, velvety texture 
much desired by consumers. Following the passage of the U.S. 
Food and Drugs Act of 1906 (the Pure Food Law), there was 
promulgated by the U. S. Department of Agriculture a series of 
so-called standards for various foods. The standards for ice- 
cream excluded gelatin, and even eggs which are essential in 
the manufacture of the “French” ice-cream. 

Upon investigating the facts J. Alexander ** found that gelatin 
not only improves the product by inhibiting the formation in 
ice-cream of sharp crystals which make the product gritty or 
sandy to the taste, but it also actually renders the fat and casein 

18 Kolloid Z. 4, 86 (1909) ; 5, 101 (1909). 


THE USES OF GLUE AND GELATIN 213 


present more digestible by preventing the formation of large 
fatty or greasy curds which are particularly hard to digest.** 

An investigation of the medical evidence showed that even 
prior to 1888 Jacobi?® had recommended the addition of gelatin 
and similar protective colloids to cows’ milk and infants’ diet. 
Experiments in vitro showed that gelatin inhibited or delayed 
the coagulation of cows’ milk by acid and by rennin, making it 
resemble mothers’ milk in this respect; and ultramicroscope ob- 
servations checked the results. Since fatty or greasy curds are 
particularly difficult to digest, the value of a protective colloid 
in ice-cream was evident. A case was brought to trial in Wash- 
ington in which the so-called government standard was over- 
thrown, and the official view regarding official legislation as to © 
how foods should be prepared, has undergone considerable modi- 
fication. Any reasonable mixture is allowed which does not 
mean fraud on the consumer or danger to the public health. 

Harper F. Zoller and Owen E. Williams ** report that with ice- 
creams containing very high percentages of evaporated skim 
milk, even the presence of gelatin may not prevent the ‘“sandi- 
ness” due to the separation of crystals of the relatively slightly 
soluble lactose. Zoller +’ has also shown that gelatin facilitates 
freezing, for gelatin solutions develop crystals more quickly than 
does pure water. Gelatin thus reduces the tendency toward 
super-cooling. 

As R. H. A. Plimmer ** remarks, the proteins must be regarded, 
biologically, as mixtures of the various amino-acids, which are 
re-shuffled in digestion and absorption. During digestion the 
proteins are hydrolyzed into their 18 or 20 constituent amino- 
acids; but the animal body cannot synthesize these acids or con- 
vert one into another, an exception being the simple glycine 
which may be formed under certain conditions. Consequently 
although animals may be maintained upon a diet whose protein 
content is replaced by the proper selection of essential amino- 
acids, the animal will fail if fed on real proteins which lack any 

4 See J. Soc. Chem. Ind. 28, 280 (1909) ; Kolloid Z. 6, 197 (1910); J. Am. 
Chem. Soc. 82, 680 (1910); Alexander and Bullowa, Arch. Pediatrics 27, 18 
(1910) ; J. Am. Med. Assn. 45, 1196 (1910). 

1 A, Jacobi, ‘The Intestinal Diseases of Infancy and Early Chilahood,” 1889. 

16 J, Agri. Research 21, 791 (1921). 


17 Ice-Cream Trade J., 1921, and private communication. 
18 J, Soc. Chem. Ind. 40, 227R (1921). 


214 GLUE AND GELATIN 


one of these essential amino-acids. To secure growth of the 
animal as well as maintenance, variety as well as quantity of 
protein is necessary. ‘This shows another danger of establishing 
a diet merely upon a fat, protein, carbohydrate, calorie basis. 
Colloidal protection, vitamines, soluble salts and protein variety 
are also essential factors. Therefore though gelatin, zein (from 
corn) and gliadin (from wheat) show marked deficiencies, they 
are nevertheless rich in many essential amino-acids. The disease 
pellagra appears to be caused by unbalanced protein diet (corn) 
and it would be interesting to see to what extent gelatin would 
help to supplement the diet in such cases. Gelatin, however, 
_ lacks the following amino-acids which are essential to nutrition— 

cystine, tryptophane, tyrosine. It is not, therefore, a com- 
plete food, nor even a complete protein food; but neither 
are most other foods. Nevertheless itis pure and easily 
digested, and the use of calves’ foot jelly and consommé for 
invalids is a long-established custom based on favorable ex- 
perience. 

In his book on “Infantilism” *® Herter describes a condition of 
arrested development, consequent upon the non-absorption of 
food and its subsequent putrefaction in the lower intestine. The 
patients excreted practically all the calcium ingested, which 
accounts for the failure of a skeletal growth; and the feces con- 
tained neutral fat, fatty acids, and soaps in marked excess, 
which indicates impaired fat absorption. Herter found that 
adding gelatin to the milk fed, caused increased absorption and 
recommends its use (loc. cit., pp. 101, 105). The gelatin evi- 
dently exercises its well-known protective and emulsostatic ac- 
tion, facilitating the digestion of fats, and thereby combating 
intestinal putrefaction; for the cream layer or fat of milk con- 
tains from 100 to 500 times as many bacteria as the whole 
milk.2° Herter observes that “in sparing protein small quan- 
tities of gelatin appear to have about as much effect as larger 
amounts.” ‘This accords with the view that gelatin functions as 
a protective colloid, for only small quantities are essential to 


make protection effective. 


19“On Infantilism from Chronic Intestinal Infection,” by C. A. Herter, The 


Macmillan Co., 1908. 
2U. S. Dept. Agri., Bull. 56, 737. 


THE USES OF GLUE AND GELATIN 215 


Food vs. Technical Gelatins. 


How shall gelatin be distinguished from glue? There is, in 
fact, no sharp distinction; for any clear, light-colored glue of 
high strength may with justice be termed a gelatin. A sharp 
line, however, must be drawn between technical gelatins intended 
for manufacturing purposes, and food gelatins intended for 
human consumption. | 

In the first place food gelatin must be free, or practically free, 
from injurious substances of all kinds. Sulphur dioxide, arsenic, 
copper and zine are the impurities most often tested for. Owing 
to refinement of analysis and the practical impossibility of elimi- 
nating all traces of metals, the following permissable limits have 
been tentatively fixed by the U. 8. Department of Agriculture 
(Bureaus of Chemistry and Animal Industry) : 


Arsenic ..... fae ee 1.4 parts per million 
(occ COO 6 6 
“ne, ek ia i aa 100.0 “ - é 


No definite figure for sulphur dioxide has been announced, al- 
though excessive amounts are considered obnoxious. The quan- 
tity is not supposed to exceed 350 parts per million, this figure 
being to cover the errors in analysis. The State of Pennsylvania 
prohibits all sulphur dioxide, but since bone and hide from ani- 
mals slaughtered under Government supervision showed appar- 
ent SO, on the official test, it is evident that due allowance must 
be made for the imperfections of analytical methods. 

Besides satisfying the chemical tests, food gelatin must be free 
from objectionable color, odor, and bacteria. It should be made 
from clean stock under clean conditions, and be kept clean 
subsequently. 


Chapter 14. 
Fish Glue and Fish Isinglass. 


Fish glue, which is usually marketed in liquid form, is made 
from fish heads, bones, and skins, that form an offal in the fish- 
ing industry. In the United States and Canada the chief sources 
of supply are the cod, haddock, cusk, hake, and pollock, the 
refuse from the salting factories yielding a large part of the 
supply. Many fish residues are now unutilized, although some 
of them, i.e. that of the mullet found in our southern waters, 
yield excellent glue. Generally whenever it is too troublesome 
or expensive to separate the glue or glue-forming stock from 
admixed “‘gurry,” salt, oil, and foreign proteins, it is more profit- 
ably converted into “chum,” which is sold as poultry food, or into 
fertilizer, which always finds a ready sale. é 

Sturgeon refuse, and the skins’ and scales of menhaden and 
herring have also been used as a source of glue. According to 
Green and Tower? one ton of menhaden yields 20 pounds of dry 
scale yielding 10144 pounds of dry gelatin (moisture content 16 
per cent.). The “stick,” obtained by concentrating the waste 
liquors of the menhaden industry, owes its adhesiveness to the 
glue present in it; but it is sold for fertilizer. According to a 
German patent (131,315) glue is made from whale blubber, after 
removing by volatile solvents the fat left after cold pressing. 
Unsuccessful attempts have been made to produce glue from 
the gray fish (Squalus acanthias) ; the dark skin pigment darkens 
the glue and the fish contains a large amount of oil and water. 
Excellent fish glue is produced on the Pacific coast of the United 
States. : 

Generally speaking, to render the extraction of fish glue profit- 
able, in addition to simplicity of handling, the stock must be 
available in abundant and steady supply. 

The fish glues of commerce are.classed as (1) head glues, (2) 


1 United States Fish Com, Bull., 1901, pp. 97-102. 
216 


FISH GLUE AND ISINGLASS 217 


bone glues, (3) skin glues, according to the stock from which they 
are made; and they are valued generally in the order given, skin 
glues being the strongest and most valuable. With the bones are 
usually included the trimmings from the salted fish. 

The manufacture of fish glue is extremely simple. The stock 
is first washed thoroughly with fresh water to remove dirt and 
blood from the fresh fish fragments, and salt from the salt fish 
offal. The old method was to boil the washed stock in open 
kettles for 10 hours with live steam, according to Lambert; or 
from 6 to 10 hours, according to Tressler,? who says that two 
runs are made. Newer methods make use of autoclaves in which 
the stock is extracted by heating under pressure for several hours, 
a steam-jacketed kettle being used. Or the stock is placed 
within the inner, perforated section of a double boiler, from 
which the glue liquor filters into the outer shell, where it can be 
drawn off continuously. Or steam and cold water may be used 
on the stock alternately until exhaustion is complete. The resi- 
due left after squeezing out the boiler residue contains 45 to 55 
per cent. protein matter and is useful for poultry food or fer- 
tilizer. 

The dilute glue liquors are strained or filter pressed, bleached 
if desired with sulphur dioxide gas, and then evaporated in open 
pans heated by closed steam coils, or in vacuum evaporators. 
After evaporation to the desired constituency, usually about 50 
per cent. water, the glue is drawn off into storage tanks or bar- 
rels, preservatives and essential oils having first been added and 
mixed in. Among the preservatives used are boric acid, phenol, 
and cresol, while sassafras and wintergreen are the most popular 
perfumes added to mask the odor of fish. 

In the production of highly clarified skin glue for photo- 
engraving work, the dilute liquors may be treated by a high- 
speed centrifuge of the Sharples type, or else forced through a — 
special pulp filter. 

Sometimes animal glues of low jelly strength are added to fish 
glue to stretch the yield, and acetic acid or salts may be added 
to depress the jellying temperature. 

A small quantity of fish glue is produced dry in the form of 
cakes or broken sheets which are very hygroscopic and readily 


2 Private communication. 


218 GLUE AND GELATIN 


soluble in cold water. The drying is a matter of difficulty, but 
the concentrated glue may be “skinned” over in pans and then 
transferred to nets to complete the drying. Oiled or waxed sur- 
faces may also prove useful. 

Properties of Fish Glue. The liquid fish glues of commerce 
are viscous liquids which gradually thicken as the temperature 
is reduced and finally gelatinize at about 5° to 10° C. (40°- 
50° F.). Ifthe glue thickens by evaporation beyond the average 
of 50 per cent. of water it usually contains, it will chill more 
readily and should therefore be warmed and reduced with water 
or acetic acid (vinegar will serve) well stirred in. 

The color of the glue varies with the stock, the method, and 
the care used. Skin glues usually are more clear and the photo- 
engraving grade is transparent. Head and bone glues are usually 
turbid and may be brown or, if bleached, a light yellow. As 
with ordinary animal glues, zinc oxide may be added to produce a 
light tone. ‘Taste and odor, which are usually very pronounced, 
depend on the same factors as color, and in addition upon the 
preservatives and essential oils added. 

Tressler* says that dry skin and fish waste glues contain 
about 1 per cent. of ash; head glues may contain from 1-65 per 
cent. He gives the following representative analysis of the ash 
of a fish skin glue: 


Per Cent. 
Ash (in water-free glue) ...:.......5+ssmemee eee 0.96 
Silica -(SiO3) « .. wea s'a cb eles oss’ halen betes ane 12.7 
Calcium Oxide (CaO)... 22.0... steer eee 10.5 
Magnesia (MgO) . 2... scenes cece ete tne Trace 
Potash and Soda (K:O and Na.O).......5.. 0.005 13.9 
Sulphur Trioxide (SO:)............ sue oe 34.0 
Phosphorous Pentoxide (POs)... .. 2s 24.9 
Chlorine (Cl) sc..c..0208, 000 sl» let ole ee an 
Ferric Oxide (Fe2Os3)........ PTET eS eh Trace 


Due to the fact that it is tenacious and dries slowly, fish glue 
will spin out long thin “spider webs,” which are popularly 
thought to be an indication of great adhesive power, and indeed 
for many purposes it serves admirably. Contrary to the prevail- 
ing opinion, however, fish glue does not begin to equal good 
animal glues for making joints. The joint strength of a com- 
mon commercial fish glue was only 260 lbs. per square inch, but 


’ Private communication. 





FISH GLUE AND ISINGLASS 219 


according to the Forest Products Laboratory (Technical Note 
F-2), high-grade skin glue should average 1,700 to 1,800 lbs. 


Isinglass. 


Isinglass is probably a corruption of the German hausenblase 
(Dutch huisenblas), literally “sturgeon’s bladder,” which has for 
centuries been the main source of the celebrated Russian isinglass, 
a product that found its way from the great fair at Nijni Nov- 
gorod to London, and the other markets of the world. Several 
varieties of the sturgeon, the beluga (Acipenser huso), the osseter 
(A. guldenstadtu), the sterlet (A. ruthenus), the common stur- 
geon (A. sturio), and the starred sturgeon or seuruga (A. stel- 
latus), as well as the catfish (Silurus glanis), and carp (Cyprinus 
carpio), which are found in the Volga and other great rivers, in 
the Caspian and Black Seas, and in the Arctic Ocean, yield 
“Russian isinglass.” The sounds of many other varieties of fish 
also appear on the market as isinglass, supplies coming from the 
East and the West Indies, Penang, Brazil, Bombay, Manila, 
Venezuela, Canada and the United States. Brazilian isinglass, 
also called “Cayenne isinglass,” is obtained from Silurus Parkeri, 
and rat’s tail isinglass is made from the cod (Morrhua vulgaris) , 
the hake (Phycis Americanus) and other fishes. The tongue 
sounds exported from Penang and Bombay are also called 
purse sounds, for they are purse-shaped with fringed edges.. 
Tongue sounds, lump and pipe isinglass, are also exported 
from Venezuela and Brazil; they are inferior to the Russian 
isinglass. | 

The fish sound or swim bladder (air bladder) is a hollow com- 
pressible sac, containing a gas (oxygen, nitrogen or carbon diox- 
ide) and is situated in the abdominal cavity. Its main use ap- 
pears to be a mechanical one, for by compressing or expanding 
it the fish can regulate its specific gravity so as to rise, to sink, 
or to remain at a certain level. 

The sound seems to be a homolog of the lung, and in some 
fishes may assume the functions of that organ. Its size varies 
greatly, but.in the sturgeon, hake, catfish and carp it is very 
large. It is made up of several layers, the inner one being thin, 
often of a silvery luster, containing crystalline substances, and 
sometimes covered with epithelium. The next layer is thick 


220 GLUE AND GELATIN 


and fibrous, and contains the collagen which yields commercial 
isinglass. 

Leaf isinglass (also known as Astrakan leaf, Saliansky leaf, 
and Samovy or Taganrog leaf) is prepared by soaking the sounds 
in hot water and removing the dirt and mucous membrane. The 
sounds are then split, and dried with exposure of the inner mem- 
brane; the outer membrane is removed by rubbing or beating. 
The unopened sound is called “pipe,” “purse” or “lump” isinglass, 
depending upon its appearance. When folded and dried they 
form ‘‘book” isinglass, and when rolled they form “ribbon” 
isinglass. ‘Trimmings are often compressed into ‘‘cake” isinglass, 
or they may be dissolved and the strained solution dried out. 
“Long staple” isinglass and “book” isinglass, which include the 
largest pieces, are most valued; a 2 per cent. solution jellies on 
cooling and yields only 0.05 per cent. of insoluble matter. 

The chief source of North American isinglass is the hake, but 
some is obtained from the cod and the sequeteague. Hake caught 
in the deep water off the Newfoundland coast have large sounds, 
one ton of fish yielding 300 to 500 sounds, weighing from 40 to 
50 pounds. Hake caught in shallow water are smaller, one ton 
yielding about 600 sounds, weighing about 30 pounds. The 
average hake sound yields about 85 per cent. of gelatin; they 
are easily separated from the backbone of the fish and are usually 
salted on the fishing vessels. Cod sounds are smaller, much 
harder to separate from the backbone (part of which often clings 
to them) and yield only about 50 per cent. of gelatin. The 
sequeteague yields a good quality of isinglass, but the production 
from this fish, once 30 tons annually, has now sunk to practically 
nothing. Experiments by White * show that the tilefish (Lopho- 
latilus chameleonticeps) also yields good isinglass. 

White also gives an account of isinglass manufacture in the 
United States, where the industry was initiated at Rockport, 
Mass., in 1821. Ribbon isinglass is the principal product. The 
sounds, after being washed and soaked until soft, are run into 
a cutting machine having a roller and a set of knives which 
reduce the sounds to small pieces. After mixing and macerating 
between a set of iron rollers, the material passes to the sheeting 
rollers, which are hollow, water-cooled, and provided with a 


*U. S. Bureau of Fisheries Document 854 (1917). 


_ FISH GLUE AND ISINGLASS _ 221 


scraper. ‘The isinglass issues from the sheeting rollers in sheets 
of variable length, 6-8 inches wide, and one eighth to one quarter 
of an inch thick. It then passes to the ribbon rollers which with- 
out widening it, stretch it to long ribbons about one sixty-fourth 
of an inch thick. These ribbons are suspended in a warm, dry 
room,’ and when dry are wound up on wooden spools. The 
yield is about 80 per cent. of the weight of sounds originally 
used. | 

A product called transparent or refined isinglass is manufac- 
tured by dissolving New England isinglass in hot water and 
spreading the solution on oiled cloth to dry. The very thin, 
transparent sheets thus formed serve as a good glue, but have a 
fishy odor. ? 

Properties of Isinglass. Pure isinglass is odorless, practically 
tasteless, tough, fibrous, and should be white with a yellowish 
tinge, opaline or translucent. The toughness, transparency, and 
flexibility of isinglass, coupled perhaps with the fact that it 
came from Russia, which was the original source of clear mica 
(muscovite or Muscovy glass), has led to much popular con- 
fusion of these terms, the mineral mica being erroneously termed 
isinglass. It contains from 15 to 20 per cent. of. water, and 
according to Mulder its percentage composition is as follows: 
Carbon, 50.76; hydrogen, 6.64; nitrogen, 18.32; oxygen and sul- 
phur, 24.69. 

Isinglass is a nearly pure collagen. When soaked in cold 
water it swells greatly without losing its organized, fibrous, 
thread-like structure. Boiling converts it into gelatin which, 
probably because of the ease of its formation, yields a very 
strong jelly. When treated with hot water, Russian isinglass 
dissolves completely, swelling uniformly to produce a whitish 
opaline jelly. This distinguishes it from gelatin, which swells 
irregularly in hot water, giving in most cases a more transparent 
solution. Adulterated or inferior isinglass may give considerable 
residue and usually has a bad odor. 

Isinglass is insoluble in alcohol, but dissolves in most dilute 
acids or alkalis. If bleached by.sulphur dioxide it may give a 
precipitate with barium chloride due to traces of sulphates. 
Russian isinglass leaves on ignition from 0.4 to 0.9 per cent. of 
reddish ash, which contains a little calcium carbonate. Gelatin, 
- on the other hand, yields at least 1.5 per cent. of ash, consisting 


222 | GLUE AND GELATIN 


mainly of calcium carbonate and phosphate, with traces of chlo- 
rides and sulphates. 

The following table prepared by F. Prollius® gives results on 
a number of specimens of Russian isinglass, and some inferior 
grades; the viscosities were determined on filtered solutions of 
1 part of isinglass dissolved in 90 parts of water: 


Residue In- 

soluble in Viscosity 
Ash Water Hot Water m 

Kind of Isinglass PerCent. PerCent. (Per Cent.) Seconds 
Astrakhan isinglass ............ 0.20 16.0 28 507 
MET occ heal ee agah 0.37 18.0 0.7 485 
" AVR seadonh Vere coe ike ie 1.20 17.0 1.0 500 
i cae ign. Are Dhara Mtoe te 0.80 19.0 3.0 491 
: Totals Meets ot Senge stoly 0.50 19.0 0.4 480 
ic RL ee ee eee 0.40 17.0 13 477 
Hamburg Sh ME Se ee a eee 1.30 19.0 2.3 470 
- Saag ae eaecetee ts EN 0.13 19.0 5.2 — 
Rolled northern fish bladder... 3.20 —- 10.8 467 
Iceland fish bladder........... 0.60 17.0 216 463 
Indian isinglass ........ ne oan 0.78 18.0 86 437 
Yellow (unknown origin)...... 2.30 17.0 156 360 


White agrees with Prollius in advising microscopic examina- 
tion for examining isinglass for adulteration with gelatin, which 
is often rolled in alternate layers with isinglass. The gelatin 
becomes more transparent on swelling and is structureless, 
whereas the isinglass shows its characteristic fibrous structure. 

The British Adhesives Research Committee ® found that the 


Hausmann numbers of isinglass do not differ appreciably from 


those of gelatins obtained from mammalian tissues. In con- 
tradistinction to glues and gelatins, isinglasses yield, upon heat- 
ing their solutions to 100°, a precipitate of coagulated albumin. 
Russian isinglass gave 4.57 per cent. coagulable protein, whilst 
Brazilian isinglass gave 13.05 per cent. 

The Hausmann numbers of the coagulable protein approxi- 
mates those of egg albumen, and are much higher than those of 
ordinary glue and gelatin. The results are: 

N percent- 


age (dry Mono- 

Albumin from wt. basis) AmideN HuminN DiaminoN aminoN 
Brazilian Isinglass ...... 14.45 6.92 2.90 20.62 69.56 
Russian Piet eS a chews 14.67 ~ 6.20 3.06 19.43 ileal 
Ogee, ces Guise oe ee 14.06 7.82 2.91 21.27%" 68.00 


Removal of the albumin materially altered the adhesive 


5 Abst. J. Chem. Soc. 45, 647 (1884). 
6 First Report, London, 1922, p. 27. 


FISH GLUE AND ISINGLASS 223 


strength of the isinglass, that of Brazilian isinglass dropping 
from 946 to 550.’ } 

Uses of Istnglass. Good edible gelatin (formerly called “pat- 
ent isinglass’”) is now produced in large quantity and at so low 
a price that it has replaced isinglass practically entirely in 
making jellies, confectionery, etc. Fish sounds especially mixed 
with tongues, are used as food, and fried cod sounds are said to 
taste like fried oysters. 

The advent of prohibition in the United States has curtailed 
one of the main uses of isinglass, for it has long been used as a 
fining or clarifying agent for cider, wines, beers, etc. It acts 
both mechanically and as a colloidal adsorbent, removing tan- 
nins and turbidity-producing particles. 

For white wine the isinglass is swollen in water and then 
beaten with wine and a little tartaric or sulphurous acid and 
strained through linen before adding to the wine. One ounce 
of isinglass usually serves to clarify 200 to 500 gallons of wine 
in 8 to 10 days. For a barrel (36 gallons) of beer about 1 ounce 
of isinglass is cut or softened in cold water containing a little 
acetic or sulphurous acid, and without dissolving is poured in and 
allowed to settle slowly through the barrel, carrying with it the 
“cloud.” One pound of isinglass will clarify 100 to 500 barrels 
of beer. Gelatin is now used also, to replace isinglass as a fining 
material. 

The formule and recipes found in old books show that isinglass 
was once largely used as an adhesive, even for postage stamps, 
envelopes and gummed paper. For these purposes it is now 
obsolete, being replaced by dextrins, gums, glues, etc. It is still 
used in leather belting cements, jewelers’ cements, and in special 
adhesives such as those used in covering iron rolls in textile mills. 

White gives the following recipe for making court plaster 
with isinglass as the adhesive: 10 grams of isinglass are dissolved 
in 120 grams of water, and one half of this solution is painted 
out on about 38 square centimeters of taffeta silk stretched on a 
frame. When this layer is dry, a second one is applied, consist- 
ing of the other half of the isinglass solution, to which has been 
added 1 gram of glycerin and-40 grams of alcohol. Finally the 
other face of the taffeta is painted out with tincture of benzoin. 


7 This is an interesting instance of cumulative protection. See J, Alexander, 
J. Ind. Eng. Chem. 19238. 


APPENDIX. 


The National Association of Glue and Gelatin Manufacturers 
(of the U.S. A.) have for some time been considering the estab- 
lishment of official methods for testing glues and gelatin, and 
while nothing has been officially published, the Technical Sec- 
tion has agreed upon the following details: 

Preparation of sample. Samples of unknown origin should be 
ground to 4 mesh before weighing. 

[Since a lot of commercial glue usually consists of a mixture 
of several boilings which may be of different grades, it is of - 
course essential that the sample be drawn with this in mind. 
With this product segregation sometimes occurs in the barrel, be- 
cause of jarring due to transportation and differences in size and 
specific gravity of the glue fragments. Therefore to secure a 
representative sample, the glue should be well mixed and a large 
sample ground in a suitable mill.] 

Concentration of test solutions. Present practice shows from 
11.11 to 15 per cent., and 1214 per cent. was tentatively fixed as 
the standard concentration for viscosity and jelly strength tests. 
This means 1 part of glue to 7 parts of water, by weight. 

Temperature of soaking. At 10° C. for at least 12 hours, or 
overnight. 

Temperature of glue solutions should not exceed 63° C._ Vis- 
cosities should be taken at 60° C. The glue should not be sub- 
jected to the heat of the water bath for more than 30 minutes. 

Cooling procedure. Allow to cool at room temperature. Then 
hold the sample at 10° C. for not less than 18 hours, nor more 
than 22 hours. 

As stated before (Chapter 12) the prevailing practice in this | 
country is to grade glues on jelly strength and on viscosity 
taken at working temperature. While the instruments have not 
yet been officially adopted, through the kindness of W. D. Rich- 
ardson there is given below a description of the construction and 
operation of the Bloom Gelometer, which represents the type of 
instrument that will probably be adopted by the Committee for 
determining jelly strength. 

An instrument of the same general type, but working on a 

different mechanical principle, is being developed by F. 8. Wil- 

liams, who is also working on a system of grading based on an 

imaginary perfect glue, whose jelly strength and viscosity, each 

assumed to be 100 per cent., are not equalled by any commercial 

glue or gelatin. To bring the measurements within the con- 
225 


226 | APPENDIX 


sumer’s range, Williams proposes as a solution of standard 
viscosity, one containing by weight 2 parts of water to 1 part 
of absolutely dry “60 per cent. glue, a grade approximately equal- 
ling Cooper’s 1144, Alexander’s 90, or Bogue’s 8. 

Regarding viscosity, W. D. Richardson informs me that the 
Committee favors a pipette or capillary flow type of viscosim- 
eter for glue testing, since it can be standardized to give 
viscosity in absolute units (centipoises). By calibrating such 
pipettes with fluids of known viscosity (e.g. glycerin, sugar 
solutions, castor oil) the variations of different instruments, with 
respect to the time interval of efflux, may be reconciled. J. R. 
Powell, of the Committee, has just (Jan., 1923) succeeded in 
calibrating pipettes in this manner. Starting with a pipette as 
near standard size as possible, he found that only one high and 
one low reading were necessary to. calibrate the instrument 
- satisfactorily. 

F. D. Williams is working on a pipette made of carefully 
ground and fitted glass sections, and having an air jacket to 
serve as a thermostat. The effluent tube is without end con- 
striction or tip, the length of the efflux tube controlling the time 
of outflow. | 

One distinct improvement suggested by the Committee, is the 
use of 8-ounce wide-mouth bottles (salt mouths) having tall, 
tight-fitting soft rubber stoppers, instead of glasses as test 
vessels. The stoppers prevent evaporation and skinning, but 
of course care must be taken to see that no water of condensa- 
tion on the upper portion of the bottle interferes with the deter- 
mination of jelly strength. The stopper is removed when making 
the several tests. 


DESCRIPTION AND WORKING DIRECTIONS OF BLOOM 
GELOMETER. 


Name: 

The name of the instrument is the Bloom Gelometer (called 
by the National Association of Glue and Gelatin Manufacturers, 
“Association Style A Gelometer’’). 


Purpose: 

The purpose of this instrument is to afford a device for deter- 
mining the jelly strength of glues and gelatins, which will be 
automatic in its action, which will be reproducible and which 
will give readings in terms of weight required to produce a 
definite depression of a plunger of definite diameter in the glue 
jelly. 


Standard Units Adopted for the Machine: oh 
The diameter of the plunger is exactly 12.7 mm. (1% inch) 
and is constructed of aluminium, the sharp lower edge being 


BLOOM—GELOMETER 
Description and Working Directions. 


HY 


a I 


HN} 


i 
3 


Ree 


IN| 


A Brass Contact Point Bracket. 

Upper Contact Point. 

Lower Contact Point. 

Wood Fibre Support for ‘A’’, 

Set Screw to Hold Adjustment Screw in 
Position. 

Adjustment Screw for ‘‘A’’. 

Pure Silver Disk (54 in. dia. & We in. 
thick). 

Electro-magnet. 

Electrical Connection from ‘‘C’’ to ‘‘S’’. 
Adjustment Screws for Adjusting Pitch of 
Clamshell Cut off ‘‘E’’. 

Guide Bar of Automatic Shot Control Mech- 
anism. 

Lower Dog. 

Dog 2. 

——— Dog 3. 

—— Dog 4. 

Upper Dog. 

Hair Spring Coil to Keep “D7” in Position, 
Acts against Electro-magnet. 

Soft Iron Bar Supporting Dogs ‘‘D1-D5’’. 
Clamshell Spout. 

Stationary Clamshell Jaw. 

Adjusting Screw to Regulate Closure. 
Weight. 

Clamshell Arm, 

Set Screw to Clamp Weight to Clamshell Arm. 
Bearing on Which Cut-off Mechanism Turns. 
Spiral Spring (No. 8 Steel Music Wire). 
Adjustable Support for Spring ‘“‘F’’, 
Thumbscrew Nut. 

Tension Spring. 

Suspended Pan and Pan Arm for Shot Re- 
ceiver, Disk ‘‘B’”’ and ‘Plunger ‘‘L’’. 

Pan Arms. 

Pan. 

Rod Attached to Pan Arms Supporting Disk 
Lal Bae 

Shot Hopper with Delivery Tube. 

Bracket to Hold Lower End of Shot Deliv- 
ery Tube. 

Upper Supporting Bracket Attached to 
Frame Support ‘“R2’’. 

Lower Supporting Bracket Attached to 
Frame Support ‘‘R2’’.: 

a Adjustable Guide Arm Attached to ‘“‘J2’’. 
Shot Receiver. 

Plunger (12.7 MM. in Diameter). 

Test Bottle. 

Elevating Platform Base. 

Platform. 

Rack and Pinion Elevating Mechanism. 
Battery Box and Batteries. 

Ordinary Telephone Condenser. 

Electrical Connections from Condenser to 
Contact Points. 

Electrical Connections from Condenser to 
Contact Points. 

Electrical Switch. 
Electrical Connections from ‘‘O” to “C”’. 
Electrical Connections from ‘‘O” to ‘“A’’. 
Base of Gelometer. 

Pillar of Gelometer. 

Fine Copper Wire Coil Making Contact 
Across from Suspended Disk to Binding Post 
on Support. 











227 


228 APPENDIX 


rounded to the slightest possible degree. The depth of plunge is 
exactly 4 mm. as determined by selected Brown and Sharpe 
gauges. 

The time period for the introduction of the shot (used for 
depressing the plunger) is kept within the limits, 2 to 5 seconds. 


General Description (Refer to figure and legend): 

The instrument is mounted on the base R, and the pillar R,. 
The adjustment stand N resting on the base R, is provided with 
a platform N, capable of being raised and lowered by the rack 
and pinion mechanism N,. Affixed to the upper end of the pillar 
R, by the bracket J,, the spring adjusting mechanism G holds 
the spring F and the plunger L, hanger and pan H, and H, re- 
spectively. At the upper part of the plunger hanger, the silver 
contact disc B is set to operate between the contact points A, 
and A,. The rod H, of the plunger hanger works through the 
adjustable guide J,, which is affixed to the bracket J,. 

Affixed to the upper end of the pillar also is the shot hopper I 
supplying shot through the clamshell cut-off E-E, to the shot 
receiver K which rests on the pan H,. The automatic shot 
control mechanism D-D,, working on the clamshell cut-off E-E, 
consists of the electro-magnet C, the soft iron bar D, carrying 
the brass dogs D,-D, respectively, and the brass guide bar D. 

The cut-off mechanism consists of the clamshell cut-off E-E,, 
the control rod E,, working on the dogs D,-D, and the counter- 
balance weight E,. E, is the bearing on which the cut-off 
mechanism turns. The entire cut-off mechanism is adjustable 
vertically on the pillar R, by means of the screws C,, this adjust- 
ment setting and adjusting the pitch of the clamshell cut-off 
E-E,. This adjustment is made when the machine is assembled 
and is permanent. 

Electric current is supplied to the electro-magnet C through 
the contact points A,-A, from the 3 volt dry battery O through 
the connections Q,, C,, S and Q,. P is a small telephone con- 
denser arranged in shunt circuit by means of the connections 
Bejan shee 

The test bottle M containing the jelly to be tested rests on 
the platform N,. 


Operation: 

The space between contact points A, and A, is adjusted as 
follows: With the current cut off by means of switch Q and 
with the silver disc B resting on contact point A,, adjustment is 
made by means of the adjustment screw A, so that the distance 
between the upper face of the silver disc B and the contact 
point A, is exactly determined. This determines the depth of 
plunge. In the case of glue and gelatin jellies, the depth of 
plunge is exactly 4 mm. as determined by the standard Brown 
and Sharpe 4 mm. gauge furnished with the instrument. 


APPENDIX 229 


Adjustment is now made of the silver disc B against contact 
point A, by turning the adjustment screw G, (which acts on 
the spring F) until the silver disc B is in lightest possible con- 
tact with contact point A,. When this point is reached, sparking 
will be noticed between the point A, and the disc B and a make 
and break vibration is set up between the soft iron bar D, and 
the core of the electro-magnet C. When this adjustment is once 
carefully made the machine stays in adjustment for some time 
but readjustment should be made occasionally. 

‘The glue or gelatin jelly (or the like) prepared in the usual 
way, or according to standard directions, is placed in the test 
bottle M and chilled to the test temperature (10° C. for 16 hours, 
or overnight, in the case of glue and gelatin jellies). The bottle 
is placed on platform N, and raised by means of the rack and 
pinion mechanism N, until the jelly is in contact with the 
plunger L and the latter is raised until the silver disc B is 
brought into light electrical contact with contact point A,. This 
point is indicated by sparking and make and break vibration 
between the soft iron bar D, and the core of the electro-magnet 
C. The shot receiver K is quickly placed on pan H, and imme- 
diately the lever E, is raised to the pre-determined position on 
one of the dogs D,-D,;. The height to which lever E, is raised 
regulates the velocity of the flow of shot. For weak jellies one 
of the lower dogs is used, for strong jellies one of the upper dogs. 
The dog selected should be such as to keep the flow of shot within 
the prescribed limit of 2-5 seconds. The finest chilled shot ob- 
tainable is used, No. 12 or finer. The raising of the lever E, 
immediately starts the flow of shot, depressing the plunger L 
into the jelly until contact is made between the silver disc B 
and contact point A,. This closes the circuit which acts on the 
electro-magnet C, moving the soft iron bar D, and withdrawing 
the support of the dog from the lever arm E,, which immediately 
falls, thus cutting off the flow of shot by closing the clamshell 
cut-off E-E,. 

The weight of shot delivered into the shot receiver K plus 
the weight of the shot receiver itself is the weight required to 
move the plunger L through the prescribed distance against the 
resistance of the jelly, and measures the jelly strength. For 
glues and gelatins this distance is exactly 4 mm. as determined 
by Brown and Sharpe gauge. 

After the combined weight is determined the shot is emptied 
back into hopper I and the machine is ready for another test. 











Author Index. 


Abderhalden, 31, 115 

Abegg, R., 129 

Acree, 93 

Adam, H. K., 81 

Aders, 31 

Adler, 112 

Alexander, J., 27, 44, 74, 75, 79, 83, 
98, 101, 110, 115, 118, 126, 174, 
175, 177, 191, 212, 223 

Allen, 118, 127, 139, 140, 205, 

Arisz, L., 76, 78, 88, 89, 103 

Arrhenius, S., 99 

Association of Official Agricultural 
Chemists, 143 

_ Atwater, W. O., 20 


Bachmann, W., 69, 75 
Baldwin, 58 
Dy: BT; 
5] 


211 


ee 


Bancroft, W. 

Bayliss, W. M., 

Beatty, 31 

Bechhold, H., 32, 34, 49, 68, 92, 98, 
106, 107, 110, 113, 129, 181 

Benedict, A. J., 41, 96 

Bennett, H. G., 72, 87, 88, 155 

Bergmann, 180 

Bernays, L. E., 18 

Berrar, 46 

Bevan, 129 

Biltz, 46 

Bingham, E. C., 99 

Blasel, 34, 60 


76, 82, 93, 100 


Bloom, 227 

Bodecker, 118 

Hogue ei, o., 53, 54, 70, 84, 101, 
102, 104, 107, 110, 129, 130, 13); 
132, 133. 134, 135, 156, 157, 174, 
176, 177, 178, 182, 186, 194, 197, 
205, 


Bottinger, 121 
_ Bourgeois, 45, 47, 118 
Bracewell, R. S., 

Bradford, S. C., 48, 71, 84 

Bragg, 27 

Brauns, D. H., 

Bridgman, Sy W., 74, 79, 86 

Briggs, 129 

British Adhesives Research Com- 
mittee, 101, 186, 222 

British Aero. Insp. ‘Dept., 185 


231 


British Eng. Standards Assoc., 
Brotman, A. G., 89 
Buerger, 119 
Bugarsky, 60 

Bugge, 46 

Bullowa, Jesse G. M., 
Burton, E. F., 87 
Biitschli, O., 68, 69 


Calvin, J. W., 43 
Cambon, 180 
Cartledge, 76 
Chercheffsky, N., 180 
Chiari, 87 

Child, W. C., 82, 204 
Chittenden, 47 

Clark, 27, 81, 130, 180 
Clayton, E. G.,) 182, 173 
Cooper-Hewitt, Peter, 167 
Couette, E., 98 

Crismer, 139 


Dakin, 35, 46 

Davey, H., 121 

Davidowsky, 174 

Davis, Clarke E., 62, 77, 81, 99, 16 
Desmouliére, A., "140 

Determan, H., 98 

Dhéré, 52 

Donnan, F. G., 38, 66, 84, 93, 95 
Dorpinghaus, 31 

Du Bois, 180 

Dumanski, 73 

Diirbeck, 141 


Einstein, Albert, 99 
Elliot, Felix A., 41, 79 
Emmett, 160 

Engler, 98 
Ewald, A., 115 


Fairbrother, 137 

Fels, J., 177 

Fenn, W. O., 86 

Field, Miss A 40, 71 

Fischer, Emil, "26, Biel ibwiies 

Fischer, Martin H. 42, rae 87, 90, 
92, 101, 108 

Fleck, H., 165 

Forest Products Laboratory, 
184, 205, 206 


186 


45, 213 


180, 


232 


Frankenheim, 68 
Freundlich, H., 109, 126 


Garrett, H., 104 

Gies, W. J., 119, 160 

Gill, 183 

Gokun, 106 

Gorgolewski, 52 

Graham, Thomas, 48, 73, 98, 125 
Grillo, 157 

Guaidukov, N., 73 

Gutbier, 126 


Hackman, 132 

Handowsky, H., 43 

Hardy, W. B., 48, 49, 64, 68, 80, 87 

Harkins, W., 27, 80, 81 

Harris, 93 

Hart, 31 

Haslam, 135 

Hatschek, E., 49, 74 

Hawk, 24, 119 

Henzold, 141 

Herold, J., 181 

Herschel, 26 

Herter, Christian A., 214 

Herzog, 112 

Hitchcock, D. I., 38, 62 

Hochstadter, Irving, 150 

Hofmeister, F., 37, 43, 87, 88, 89, 
115, 160 

Holborn, 52 

Holmes, H. N., 82 

Hopp, G., 183 

Hoppe-Seyler, 113 

Houseman, P. A., 183 

Hunt, G. M., 206 


Illert, G., 151, 159, 168 


Jacobi, Abraham, 213 
Jones, W. L., 206 
Jordis, E., 71 


Kahrs, Friman, 196 
Keller, R., 65 

Kelly, 115 

Kern, E. J., 95 
Kieser, K., 174 

Kind, Maurice, 167 
Kissling, 131, 174, 194 
Kohlrausch, 52 
Kohlschitter, V., 210 
Kossel, 31 

Kriger, 35, 45 
Krukenberg, 118 
Kuhn, A., 65, 84 
Kutscher, 31 


Lambert, 165 
Langmuir, I., 27, 80 


AUTHOR INDEX 


Laqueur, E., 33 

La Wall, 211 

Leeds, 57 

Leffmann, H., 211 

Lehner, Victor, 138 

Levene, P. A., 31 

Levites, 8S. J., 104, 106, 124 

Liebermann, 60 

Liesegang, R. E., 48, 113 

Lillie, R. S., 91, 109 

Lipowitz, 121 

Little, A. D., 142 

Lloyd, Dorothy Jordan, 35, 38, 46, 
51, 60, 61, 70, 71, 83, 85, 87, 107 

Lloyd, John Uri, 29 

Loeb, Jacques, 32, 36, 46, 51, 53, 59, 
66, 71, 81, 82, 84, 91, 96, 102, 
107, 108, 109, 130, 131 

Low, W. H., 175 

Lowndes, A. G., 210 

Liideking, C., 87 

Lumiere, 124, 125, 126, 128, 129 

Liippo-Cramer, 127. 


MacMichael, 98 
McBain, J. W., 81, 109 
McKee, C. R., 205 
Mathews, A. P., 120 
Matula, J., 34, 60 
Mayes, 60, 61 
Mecklenberg, W., 76, 79 
Mehler. 46 

Menz, W., 75, 142 
Meunier, L., 124 
Michaelis, 57 

Moeller, W., 70, 84 
Morochowetz, 118 
Morner, 31, 111, 118, 119 
Miihlenstein, E., 85 
Mulder, 47, 118, 121, 127 
Murray, 12, 15 


Nageli, 68 

Nat’l Assoc. of Glue & Gel. Mfrs., 
220 

Namias, 123 

Newberry, Percy E., frontispiece 

Northrup, J., 14, 57, 139 


Oakes, Earle T., 62, 77, 99, 175 

Okuda, Y., 328 

Oppenheimer, 111 

Oryng, T., 91 

Ostwald, Wilhelm, 98 

Ostwald, Wolfgang, 25, 43, 44, 66, 
70, 73, 74, 89, 90, 91, 100, 101, 
106, 108 


Pallser, 111 
Pauli, Wolfgang, 32, 43, 68, 87, 108 


AUTHOR INDEX 


Pekarskaja, G., 112 

Perrin, J.,. 26, 64 

Phmmer, R. H. A., 31, 80, 114, 213 

Poetschke, P., 211 

Poinearé, 29 

Pope, 211 

Powell, Rufus W., 15, 16 

Prager, W., 160 

Prescott, 29 

Prollius, F., 222 

Procter, H. R., 46, 53, 72, 84, 93, 108, 
160 


Quincke, G., 87 


Rakusin, M. A., 110, 111 

Raschig, H., 211 

Ricevuto, 122 

Richardson, W. D., 225, 226 

Rideal, S., 121, 128 

Roberts, 27 

Robertson, T. Brailsford, 25, 33, 35, 
61, 100, 101 

Rosellini, frontispiece, 14 

Rudeloff, J., 177 


Sadikoff, 47 

Sakur, O., 33 

Samec, M., 33 

Sammett, C. F., 181 

Sauer, E., 126, 171, 172 

Scarpa, O., 86 

Schelling, 126 

Scherrer, P., 27, 84 

Schmidt, oe 141 

Schroeder, 157 

Schroeder, von, see von Schroeder 

Schryver, 'S. Bevis 

Schiitzenberger, 45, 47, 118 

Schwartz, 31 

Schwendener, S., 68 

Schwerin, 165 

Seeman, 140 

Setterberg, 183 

Seyewetz, 124, 125, 126, 128, 129 

Seymour-Jones, F. L., 114 

Sheppard, S. E., 41, 56, 79, 96, 175, 
176, 181, 208 

Skita, 31 

Skraup, 30, 31 

Smith, C. R., 38, 40, 46, 47, 49, 52, 
ba, 59, 77, 91, 92, 107, 142, 176, 
182, 191, 209 » 

Solley, 47 

Sorensen, 36 

Spiro, K, 43, 87, 90 

Ssadikow, W. S., 116 

Stearns, A. E., 65, 84, 91, 94 

Stelling, 132 

Stewart, 128 


- Wilson, J. A., 46, 53, 


233 


Stiasny, E., 124, 127 
Stokes, 127 

Sutherland, N., 69 

Swan, 137 

Sweet, S. S., 79, 96, 176, 181 


Taffel, Alan, 96 

Thiele, Ludwig, 155, 156, 163, 171 
Thomas, A. W., 58, 114, 115 
Thompson, F. C., 73 

Tian, A., 85 

Tolman, R. C., 65, 84, 91, 94 
Town, G. G., 180 

Tressler, 218 

Trotman, S. R., 132, 160 
Triax, To Ras205 

Turner, W. E. S., 78 


Upson, F. W., 43 


van Bemmelen, 69, 127 

van der Lingen, J. S., 85 

van der Waals, 26 

van Name, 47 

van Slyke, 46 

Varga, G., 78 

von Biehler, 30, 31 

von Gaza, W., 114 

von Lepowski, 75 

von Nageli, 68 

von Mehring, 118 

von Paal, 45 

von Schmoluchowski, M., 99 

von Schroeder, P., 48, 104, 105, 106. 
107, 111, 129 

von Weimarn, 29, 47, 75 


Watson, H. J., 132 
Wiedemann, E., 87 
Weidenbusch, 183 
Wells, P. E., 28 

White, 220 

Wilkinson, Sir J. W., 14 
Williams, F. §., 225, 226 
Williams, Owen E., 213 
Williams, R., 121 

84, 98, 94, 95 
Wilson, W. H., 94 
Winkelblech, K., 82, 182 
Wintegren, 35, 45 
Wislicenas, 114 
Witzemann, E. J., 81 
Wolff, A., 160 

Wood. Joi, 121 

Wood, T. B., 48, 64, 139 


Ziegler, J., 106, 181 

Zlobicki, 138 

Zoller, Harper F., 213 

Zsigmondy, R., 68, 69, 74, 75, 141, 
142 


Subject Index. 


Acid phosphate, 156 

Acrylic aldehyd, 128, 211 
Alanine, 30 

Albuminoids, 20, 22 

Albumins, 21 

Alexander’s grades, 191 

Alum, action of, 125, 129 
Aluminium salts, action of, 125 
p-Amidophenol, 124 
Antiseptics, 167 

Arginine, 31 

Arsenic in gelatin, 143 

Ash in gelatin and glue, 143, 194 
Aspartic acid, 30 


Bacteriology, 211 
Bergman process, 157 
Bichromates, action of, 124 
Bleaching liquors, 165 
Bloom gelometer, 226 
Blow-down processes, 172 
Bogue’s grades, 191 
Boiling apparatus and methods, 160 
Bone, formation of, 113 
Bone stock, 152, 153, 154 
Bones, extraction of, 155 
leaching of, 156 
Bromine, 128 
Brownian motion, 75, 101 


Cadet’s test, 179 
Cambon’s fusiometer, 180 
Chilling, 167 
Chitin, 22, 119 
Chitinoids, 22 
Chlorine, 128 
Chlorophenol, 124 
Chonchiolin, 22 
Chondrigen, 22, 117 
Chondrin, 22, 117 
Chondriotic acid, 119 
Chondroitin, 23 
Chondroitin-sulphuric acid, 23, 112, 
119 


Chondromucoid, 119 
Chondroproteins, 23 
Chondrosin, 119 
Chromoproteins, 24 
Chrome alum, 123 
Chrome compounds, 123 


Chromic oxid, 124 
Clarification of liquors, 164 
Collagen, 22, 113 
Colloidal Behavior, Loeb’s Theory 
of, 36 

Colloidality, maximum zone of, 109 
Colloids, diffusion of, 48 

Graham’s definition of, 49 
Colloidoscope, 98 
Colorimetric indicators, 130 
Comparative set, 189 
Compositions, 208 
Cooper’s grades, 191 
Cooper-Hewitt machine, 167 
Copper in gelatin, 145 
Crazing of glue, 135 
Cutting, 168 


Dentelles, 158 

Detanning leather, 159 

Diffusible nitrogen test, 136 

Donnan equilibrium, 38, 66, 93, 103, 
108 

Drying, 168 

Durol, 124 


Fdestin, 21 
Egyptian’s -using glue, frontispiece, 
14 


Engler’s viscosimeter, 177 
Erythrin reaction, 117 
Evaporation, 166 


Fels test, 177 

Filtration of liquors, 164 
Finger test, 189 

Fish glue, 216 

Flexible glues, 208 
Fluidity, 99 

Foam, 188 

Food gelatin, 212, 215 
Forces, classification of, 28 
Formaldehyd, action of, 128 
Formo-gelatin, 129, 211 
Fusiometer, 180 


G-acid, 124 

Gallotannic acid, 124 

Gelatin, action of tanning sub- 
stances on, 121 


234 





SUBJECT INDEX 


Gelatin, as a chemical entity, 51 
as a food, 212 
ash-free, 40 
ash in, 143 
A.O.A.C. tests, 143 
arsenic in, 143 
chemical examination of, 130 
chemical structure of, 31 
copper in, 145, 147 
crystallization of, 47 
deaminization of, 34 
defined, 11 
det. of nitrogen in, 132 
det. of total acidity, 131 
detection of, 140 
enol-form, 35 
gold number of, 141 
heat of swelling, 87 
hydration of, 44 
in ice cream, 212 
jellies, structure of, 68 
keto-form, 35 
lead in, 145 
molecular structure of, 25 
molecular weight of, 45 
package, 172 
philology, 12 
phosphorus in, 143 

_ polariscopic constants of, 147 
polariscopic examination of, 77 
reactions of, 138 
sheet, 172 
solutions, structure of, 68 
statistics, 16 
sulphur dioxide in, 148 
swelling of, 44, 87 
tannate of, 122, 138 
testing of, 173, 225 
thermal expansion of, 96 
titration curve of, 60 


ultramicroscopic examination of, 


uses of, 100 
viscosity of, 98 
x-ray spectrograph of, 27 
zinc in, 146 
Gelometer, Bloom’s, 226 
Gliadin, 21 
Globulins, 21 
Glucoproteins, 24 
Glucosamin, 23 
Glue, chemical examination of; 130 
cracking of, 172 
crazing of, 135 
defined, 11 
detection of, 140, 143 
Egyptians using, frontispiece, 14 
history, 13 
hydrolysis of, 160 
TORT. of, 151 


235 


Glue, philology, 12 
proper methods of using, 200 
statistics, 16 
testing of, 173 
total acidity, 131 
uses of, 200 

Glue stock, 152 

Glutamic acid, 30 

Glutelins, 21 

Glycine, 30 

Glycoproteins, 23 

Glycuronic acid, 23 

Gold number, 141 

Grease, 188 


.Grease, yield of, 172 


Grillo-Schroeder process, 157 


Halogens, 127 
Hectographs, 208 
Hematin, 21 
Hemoglobin, 21, 23 
Hide stock, 152, 153, 158 
Histidine, 30 
Hofmeister series, 37, 58, 90 
Hordein, 21 

Hydrochinone, 124 

Hydrogen ion concentration, 56, 130 
Hygrometric test, 179 


Ice cream, use of gelatin in, 212 
Immersion test, 179 
Indicators, colorimetric, 130 
Infraproteins, 25 
Tron salts, action of, 126 
Isinglass, 22, 216, 219 

uses of, 223 
Tsocolloidism, 82 


Jellies, structure of, 68, 83 

Jelly strength, 174 

Joining (see also Strength test), 204 
Joint test, 182, 189 


Kahr’s tests, 196 
Karaya gum, 109 
Keratins, 22 

Kind’s machine, 167 


Laboratory test series, 186 
Lead azide, 210 

Lead in gelatin, 145 
Leather belting, 206 
Lecithoproteins, 23 
Leucine, 30 

Lignin, 114 

Lyotrope series, 88, 90 


MacMichael viscosimeter, 197 
Mathematics, dangers of, 99 
Melting point, 180 


236 


Metaproteins, 24 

Moisture, det. of, 194 
Monochlorhydroquinone, 124 
Mucins, 23, 117 

Mucoids, 23 

Myosan, 24 

Myosin, 24 


Neredol, 124 
Neurokeratin, 22 
Nitrogen, det. of, 132 
Nucleoproteins, 23 


Occlusion theory, 102, 109 
Open tank, 161° - 
Orcine, 124 

Oryzenin, 21 

Ossein, 113, 157 
Ossification, 113 
Oxyproline, 31 


Paper boxes, 206 

Passburg system, 168 

Peptides, 24 

Peptones, 24 

Phenol, 124 

Phenylalanine, 30 

Phosphomolybdic acid, 127 

Phosphoproteins, 23 

Phosphorus in gelatin, 148 

Phosphotungstic acid, 127 

Photographic gelatin, 208 

Photography, special test of gelatin 

for, 208 

Picric acid, 124 

Pipette, Alexander’s, 177 

Plasticity, 100 

Polariscopic test, 192 

Polypeptides, 25 

Pressure in joining, 205 

Pressure tank, 162 

_Printer’s rollers, 208 

Prolamines, 21 

Proline, 31 

Prosthetic group, 23 

Protamines, 21 

Proteans, 23 

Proteins, alcohol-soluble, 24 
classification of, 21 
coagulated, 24 

Proteoses, 24 

Pyrocatechin, 124 

Pyrogallic acid, 124 


Reaction, 187 
Report sheet, 195 


SUBJECT INDEX 


Resorcin, 124 
Ruf system, 168 
Running test, 177 


Salmine, 21 

Schattermann’s test, 179 
Scleroproteins, 20, 22 
Selenium oxychloride, 138 
Sericin, 22 

Setting point, 182 

Sewage disposal, 154 

Shear test, 182 

Silicic acid, action of, 125 
Sinew stock, 152, 153 

Sizing, 206 

Smith’s polariscopic test, 192 
Spongin, 22 . 
Spreading, 168 

Standard glues, 190 
Standards, 190 

Stiffening, 206 

Stoke’s method, 127 

Strength test, 182 

Sulphur dioxide in, 148 


Tanned stock, 152, 159 

Tannin, 121 

Tanning substances, 121 

Technical gelatin, 215 
Tendocollagen, 116 

Testing, 173 

Test series, 183 

Tests, discussion of results of, 197 
Thiogelatin, 116 
Thionylxanthoglutin, 117 


Ultrafiltration, 49 
Ultramicroscopic examination, 75 
U. 8S. Navy specifications, 205 
Uranium salts, action of, 127 


Veneers, 206 
Viscosimeters, 98, 177 
Viscosity test, 177, 188 
Viscosity, 98, 100 


Washers, 159 

Water absorption test, 179 

Water supply, 154 

Wood joints (see also Strength 
test), 204 

Wounds, healing of, 114 


Yields of glue stock, 172 


Zein, 21 
Zine in gelatin, 146, 147 























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